If there is one painting, John Bellany says, that could sum up how he fought depression, illness, alcoholism, heart attacks and tragedy – and survived them all – it would be this.

He is showing me a new picture. It is a fresh self-portrait and it is a striking image, featuring symbols and icons that are familiar to anyone who has seen any of Bellany's art during his epic voyage through life. But there is a sense of stillness and calm. There is a dash of humour: in the painting, Bellany is dressed in black tie because he has often said he can paint with no splatter. There is that familiar haar of lingering horror: in the demonic figure lurking at his elbow, and in the smeared wings of the painting, which he completed on June 18 this year, his 70th birthday.

And, yes, there are emblems of the North Sea, the constant shifting backdrop to his tumultuous life. So there is a pensive seabird, a malevolent lobster, the hint of a boat's stern. There is a waiting accordion and some distant palm trees. He has painted such emblems hundreds of times, but in the new self-portrait, they – and he – seem ragged, exhausted, but still vital.

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Tired and sick, the man himself is facing me in his studio at home in Cambridgeshire. He is bearded and gravel voiced, Port Seton – his birthplace and ever-present muse – still audible in his accent, an East Lothian edge as thick as a rind of salted herring. We both lean over a copy of the self-portrait in the wide, light and airy studio in this quiet home, one of three he owns, including a house in Barga, Italy, and a flat in Edinburgh. Elsewhere in the house, in the kitchen, his indefatigable wife Helen, whom he has married twice, is preparing pea soup, bread and cheese for our lunch.

"It's like one of Rembrandt's last paintings," he says. "It's about approaching the Almighty, because really you are in that netherworld between life and death when you are 70. You are struck by so many multiple emotions that you don't really know whether you are on your head or your heels at some moments. In some moments you are so delighted that you have eventually made the threescore years and 10. So that is why I included the accordion – it is waiting there to be played in celebration. There are figures in the background who will remain anonymous. There is a mood, and it's hard to capture, of silence. It's inward-looking and silent."

His fingers, stained with yellow and blue paint, move over the image. "See? He, I, am holding the brushes, not as strongly as I used to hold them, but still holding them." He sighs heavily. "It's not that the end is near, it's that the end is coming, and you don't know many years you have got on this earth, but the birthday is a hint that your life is running out." Which adds even greater meaning to the forthcoming, milestone show at the Scottish National Gallery, John Bellany: A Passion For Life.

Bellany, 24 years on from what was, at the time, an experimental liver transplant, thinks he may live another 10 years. But in the mornings Helen, like a mother with a new baby, puts a hand over his mouth to check he is still breathing. Bellany – who has also survived three heart attacks, one dramatically outside Glasgow's Hilton hotel before a show at the Mitchell Library in 2005, pneumonia and six years of harsh depression – is optimistic.

"Let's hope I have another decade. But it's not guaranteed. And yet the feeling I get most of all is is: 'I haven't done enough yet.' There's more creative work to be done, but will I have time?"

His son, Paul, says, "We've given up saying: 'It is pretty soon that he will go.' He could be around for another 15 years. Even at his worst, he would pull a truck by his teeth to get to a blank canvass."

Back with Bellany, I point to the lobster, which sits sullenly at the base of the new self-portrait. What does it mean? "The lobster there," he says, pointing to his trademark shellfish, "is saying, 'Come on John, get on with finishing this painting.'"

John Bellany is grumpy when we arrive early. Considered one of the greatest living Scottish painters, the artist of boats and lobsters and fish, of compelling darkness and newfound peace, is working in his studio when I knock on his door.

So Bellany, still with a leonine, if aged, frame, answers the door gruffly and ushers me to the kitchen to wait while he finishes his work.

Eventually I am led into his studio, which is packed with canvasses, posters, photographs and paint. The photographer snaps away, but is suddenly told to leave, as he is disturbing the artist's train of thought.

Bellany is thinking about the epic voyage of his life, and how reaching 70 at all, after a life replete with drama, success and sorrow, brushes with death, catastrophic drinking and resounding resurrections, is a miracle, albeit one forged by medical science and an indomitable will. Though successful as a painter ever since he left home to attend Edinburgh College of Art in 1960, it was drink that nearly killed Bellany. Fuelled by the mental conflicts stemming in part from his deeply religious upbringing, the collapse of his 1964 marriage to Helen and the death of his second wife Juliet in 1985, the same year his father passed away, his drinking was so damaging that he was told that if he didn't have a liver transplant he was going to die.

He underwent that traumatic operation in 1988. It was the beginning of a second life for the painter, who had found success at the London College of Art in the late 1960s. Bellany, who had an early National Galleries retrospective in 1986 and was awarded a CBE in 1994, was showered with plaudits, sales from avid collectors and critical kudos.

But he wants to talk about none of that. Instead he wants to talk about his paintings, and his childhood. Large models of ships are in his studio window, all bearing the letters LH, the sign that marks fishing boats registered in Leith. We are surrounded by large, colourful canvasses, oil paintings of boats, of fish, of women and men, of lobsters and carrion. We sit on chairs, bathed in clear light from large windows on two sides. There is a fireplace at one end of the room, cluttered with cards, notes, mementos and photographs, including one with Alex Salmond, another with one of his friends, David Bowie.

It is clear Bellany is not well. He says as much. Indeed, while waiting in the kitchen before the artist is ready to speak, Helen tells me he suffers from hallucinations, depression and remorse, paranoia and waking nightmares. He is, she says, "tortured mentally". His eyesight also sometimes fails him, as he suffers from macular degeneration, but he is able to watch television, especially football – he supports both Hibs and Chelsea – and he can paint. "His eyesight seems to ebb and flow," she says.

Helen, from Golspie in Sutherland, met Bellany at Edinburgh College of Art, where she was studying drawing and painting. They first set eyes on each other at the freshers week concert, and got talking a year later at evening classes. After a week, in December 1962 they had their first date and soon moved in together.

"And that was it," she says. "I was 19 and he was 20." The couple, whose life is a remarkable love story in its own right, parted in 1974 but got back together in 1984 and married again two years later, with their three children – Paul, Jonathan and Anya – as the only guests.

Helen says she realised she and John could not co-exist if they were both artists so she gave up her art. She is now writing her autobiography. It will be a riveting tale. Not only her two marriages to Bellany, but the troubles of her children – both sons became highly involved in the far-right music scene, before extricating themselves. "I've written about 100,000 words already and I am really enjoying it," she says. "I am crying and laughing as I write. It's very long – it is like War And Peace. It has every facet of my life with John."

As for her husband, Bellany says he feels "fortunate that I had such a wonderful wife who was as in love with art as I was. She has become part of me".

The artist himself, now ready for our interview, is dressed in a black and silver striped jacket, neatly pressed chinos, heavy trainers and a scarf in green, yellow, red and black stripes. His eyes are like almonds, heavily lidded, and he has light hair and a full beard of brown and silver. These days, he says, he is mainly thinking about that childhood in Port Seton and Eyemouth. It dominates his thoughts and artistic dreams. It was a paradise, he says, and one he wants to regain.

Over two hours, he tells me stories of sailors and fishermen, the histories of individual boats and captains. He rhapsodises about the closeness of the post-war fishing community he grew up in, son to a fisherman, Richard, and Nancy Bellany, and all that it taught him.

"They were such dear people, the salt of the earth. I loved them so much it was unbelievable," he says. Boats and fishermen, fish guts and shellfish, have dominated the symbolic language of his painting ever since that childhood.

He spent a lot of time in Eyemouth. "It was filled with wonder for me," he says. "There were so many things happening, apart from the fishing, that your mind boggled. I was absorbing this, and it was a happy community. It was isolated from the A1 and the route to England. That kind of closeness has been kept, really to the present day.

"There were incredible things to see – the shells and fishes were just lying there, the nets were just lying there. Because it was the high point of the fishing industry, there were so many boats you could walk across the harbour, from boat to boat. It was such an honour to be fishermen then. Everyone looked up to you. But a lot of people were lost at sea, more than you would think."

The strict Calvinist faith of his boyhood – the Church was the fulcrum of the community, and his family attended three times on Sunday – has waxed and waned over the years, he says, but the memories have not. "In fact, what's happened is that I have come full circle – that is what I have been painting for the past two years, maybe even the past three years, the boats that I drew on a big ledger when I was a child." Indeed, the studio is full of large canvasses of harbours and boats. "Eyemouth and Port Seton in my youth are always to the forefront of my mind these days. I can't get away from it, and don't know what to do about it. It was a kind of paradise. It wasn't a paradise lost, it has been regained."

Bellany relishes his reference to Milton. Indeed, Christian faith, he says, however challenged and challenging, has been his "guiding light" over the past seven decades. "I just think if you have that guiding light, you will survive whatever comes your way, as I have done – all these transplants, pneumonia, three heart attacks, two strokes, all these things.

"I don't believe it was just me getting through these things, I think there was another influence pulling the strings. It might sound silly and stupid and people can say that I am silly, but that is always behind my way of thinking."

He believes in a very old dichotomy, reinforced by his visit to Buchanwald concentration camp in Germany in 1967. "There is an entity we can call evil and there is an entity we can call good," he once said.

Bellany's painting style has rarely changed in recent decades, being a blend of realism, surrealism and expressionism that has its own language, signs and emblems. The National Gallery show, more than 60 paintings, features the spread of his career, from boatbuilders and triptychs to landscapes and beautiful drawings, watercolours (which he paints at night) and prints. There is an overwhelming sense that the retrospective has kept Bellany going – both physically and mentally – this year.

"There are no lies in my imagery," he says. "It is straight from the soul, and you cannot get better than that. Anyone who tries to do painting, they have to find their own vision. People have visions of this, that and the next thing, but if there is not that vital spark in the painting, it's a failure. It's very difficult to get that spark going and difficult to get into your work."

How does this process work in him, I ask. It just happens, he says. He has visions – "good, bad, evil, not evil, happiness and joy too" – and through his brush, evokes them on canvas. Even through the darkness of recent times. "Some days you wake up so depressed," he says. "I have just come out of a six-year depression, and you have to deal with that, and the paintings just got darker and darker."

Bellany's voice becomes scratchier – he is getting tired. "Being in despair - that is absolute hell. And you cannot get out. You are trying your best to get out of hell, and all these things are dragging you back down. All these awful things. And you make a painting and you think: 'Nobody is ever going to look at this.' But they do."

So what keeps him going? His son Paul thinks it is the daily act of painting itself. His wife hints at an invincible willpower which is echoed in the title of the National Gallery show. The painter? "It's just my spirit. That's all I can say. As long as you are true to yourself.

"It's the old Hugh MacDiarmid quote [from A Drunk Man Looks At The Thistle]: 'And let the lesson be – to be yersel's/ Ye needna fash gin it's to be ocht else.' And if you have that spirit – and I had such an adoration for painting that it was the very core of my body – then that will help."

John Bellany is beginning to talk in the past tense and he looks very tired. With that, Helen calls for lunch, and, it seems, with some relief, the artist calls the interview to a halt, and we stop talking.

John Bellany: A Passion For Life is at the Scottish National Gallery, Edinburgh, from November 17-January 27. Admission is free. Visit www.nationalgalleries.org.