SATURDAY, an empty ballroom in an Essex hotel. Amy Macdonald takes a seat and peeps wearily through her fringe. She's had just three hours sleep in the last 24 - on a tour bus travelling overnight from the Netherlands - and, though dressed to trill in her tartan stage gear, I don't think she'd dance if I asked her. Still, in a couple of hours she'll be doing just that, stomping through a mid-afternoon set at the V Festival near Chelmsford and, as she surveys the crowd, perhaps grinning inwardly at the Scotland the Brave flag held aloft near the front.

It will be her third festival appearance in as many days - before the Netherlands she performed in Belgium and on Sunday there's a second V Festival appearance scheduled in Staffordshire. On Monday she flies to Norway for more live promotional work. In a month's time, she returns to America where she hopes to land some important television slots. An appearance on fellow Scot Craig Ferguson's chat show looks certain, but heavyweights Jay Leno and David Letterman are also interested in booking the girl from Bishopbriggs.

But before all that she has two other important dates to look forward to: her 21st birthday, which falls tomorrow; and a triumphant return to Scotland on Friday for the three-day Hydro Connect Festival in Inverary. Macdonald opened the same festival last year, but this time she's third from top of the bill on the main stage. It's a rapid ascent in anyone's book. As she did in 2007, her mother and some friends will be coming along for the craic.

So much for the present and the future. A couple of months ago, meanwhile, in a record shop somewhere, somebody put their hand in their pocket and bought the millionth copy of Macdonald's debut album, This Is The Life.

"I can't remember where I was when I heard, but it was really exciting," she tells me as she unscrews the cap on a bottle of mineral water. "I knew we were getting close, but not as close as that. It was brilliant news. It's very rewarding to know that all the hard work and sleepless nights pay off. When something like that happens you realise that's why you're doing it, that's why I'm getting up at three in the morning."

Her blog offers a little more clarity on time and place. "1 Million!!!" screams the headline on her entry for June 10, written from home. "I was informed earlier on today that I have now sold 1 Million albums and counting of course :-)"

She was right to smile. Released a year ago, This Is The Life went to number one in the UK album charts and has hardly been out of the top 40 since. It has also topped the charts in the Netherlands, Switzerland, Belgium and Denmark and reached number five in Germany. Last week it was finally released in America, the territory that represents the Holy Grail for any ambitious young singer-songwriter. And Amy Macdonald is certainly that.

Still, the touring and promotional work has been relentless since This Is The Life was released and will continue that way into 2009, when Macdonald takes off for Australia, New Zealand and Japan. It's possible she hasn't even reached the schedule's mid-point yet, so when I ask her about new work and a second album she tenses. Now the tiredness in her voice is mingled with a little frustration.

"I'm not writing any songs right now. I don't have the time. I don't have the energy," she says. "The things that inspire me are everyday life and, when you're doing what I'm doing, you don't have an everyday life. People can say Oh but you travel the world', but in reality I see absolutely nothing. I'm going to Norway on Monday and I'll see the airport and a hotel and that's it. And when you do this day in, day out, when you have little sleep, it doesn't inspire you."

This just spills out, not a complaint exactly, more a statement of fact. Macdonald has become a sort of commercial traveller, always in the air or about to be, always performing or about to. "I know that I need to go home and have a normal life because that's what inspires me to write songs," she adds.

Home has other appeals too, as it now also includes a fiancé - English footballer Steve Lovell, currently plying his trade with Falkirk after leaving Aberdeen over the summer. Macdonald, for her part, is a Rangers fan, though for how much longer remains to be seen: she's been as unimpressed by the on-field displays as she has been dismayed by the off-field behaviour. "I've given up on that shambles," she says, though I don't suppose she really means it. What is genuine is her condemnation of the fans who rioted in Manchester after May's UEFA Cup final defeat to Zenit St Petersburg.

"They're not proper fans, they're just there for the hell of it, to start something," she says. "And that's just such a blight on our country, it makes us look like absolute idiots, like we could start a fight in an empty room. That part of it really annoyed me. I look at it and think it's absolutely pathetic how anybody could go somewhere and react like that."

Macdonald announced her engagement to Lovell on her blog entry for July 18, adding: "I'm happier than I've ever been. I don't really feel the need to talk to the press about things like this as it's my private life and I'd like to keep it that way but thanks to everyone for their lovely messages."

In person she's as good as her word. "We're not really talking about that right now," she says when I inquire about the coming nuptials. "It was just one of those things. I didn't expect it, but it was lovely and we're both really, really happy." But she will continue to perform her song Footballer's Wife, she says, enjoying the irony of a lyric which rips into WAG culture.

"I'm not the kind of person that I'm talking about," she stresses. "I'm talking about your Jade Goodys, who goes on Big Brother and is now a massive star, and your Coleen McLoughlins, that are now incredibly famous because of who they're with. That isn't me at all, so I still totally stand by everything I said. It might seem a bit strange because I'm going to be married to a footballer, but I still do my own thing and I've got to where I am now because of my hard work rather than anybody else's."

That hard work will continue, but everything else in Amy Macdonald's life is changing fast. It will change even faster if the early promise she has shown in the American market continues. She has already performed radio sessions and live gigs in New York, Boston, Philadelphia, Detroit, Portland, Boulder and - importantly - Nashville, and has taken to including a cover of Bruce Springsteen's Dancing In The Dark in her live set. September's tour will take her back to New York as well as Chicago and, for the first time, Los Angeles. If she impresses in the City of Angels - or, more specifically, in its television studios - she stands to see that million records sold become two or three or four. In America, perhaps even on prime time television, she will tell her story again: how she bought Travis's The Man Who album with a tenner her grandmother gave her on holiday in Rothesay; how she started writing songs on a guitar aged 12; how she busked round Glasgow cafes from the age of 15 onwards; how she wrote Youth Of Today when she was barely into her teens. And, finally, how Poison Prince is an ode of sorts to Pete Doherty. Whatever else we think about Doherty, there is no denying that he is - or was - a talented songwriter. He has made a present of himself to the voracious tabloid editors, but to young teenagers such as Amy Macdonald who discovered his former band The Libertines, he has gifted something a great deal more important - excitement, expression, permission to be poetical, to sing in the demotic. He lit fires, basically, and while not all of them took, the one in Macdonald did.

She admits that today he is a mess. "I've seen Babyshambles a couple of times, but it's not that awe-inspiring, just him clattering around a stage drunk and off his face," she says. But she also recalls the other Pete Doherty, the one who once performed an after-hours acoustic set in Glasgow's Barfly venue. "The person who sat there with his acoustic guitar singing his poems and songs is the person who inspired me and it's sad because people can't see it. They mock you for saying he's an influence but, if they were to see the talent behind all the charade, they would maybe understand a bit better."

Travis, of course, were the other big influence. Macdonald dragged her mother to T in the Park so that she could see them play and it was their winning pop-rock sound that formed her earliest musical template. Her devotion to her blog and her MySpace page are in part a result of her desire to let her own fans communicate with her in a way she was never able to with Travis.

"When I was young and looking up to Travis it felt like me and them, whereas now things have levelled out. People can get in touch with you and they can talk to you and tell you what your music does for them."

She talks too about a chain developing, facilitated by the internet, in which performers can interact with fans, influencing them in turn and inspiring them to write their own songs. "It's cool to watch the little chain developing," she says.

In conversation Macdonald is clever, passionate and interesting. America will lap her up if it knows what's good for it, but in this country and in Europe the narrative has been written, the tale told. We've seen and enjoyed the first act of her story and now we await the second, the maturing of a songwriter who has shown the talent of the teenage girl but hasn't yet let us glimpse the woman. The V Festival crowd is treated to one new song - it's called Troubled Soul - but there's little else to indicate the direction in which her music is headed. The career trajectory remains true, but what about the work?

Time to write may be lacking, but when it does come she says she has no intention of altering how she does it or what she writes about. It's certainly true that her style and sound are classic enough to fit any age. But she admits that along with the expected pressures, success has brought some unexpected ones too.

"Before I was signed I could write a song and I'd know if it was good or not. Now when I do it, I have no idea if it's good, rubbish or indifferent. I just can't tell any more. You wonder if people are going to like it, if it's going to be good enough. I just can't tell."

We talk about the actual craft of song and verse writing. I ask Macdonald if she feels she's ever really opened up emotionally in a lyric.

"I've had a really happy life and a happy existence, so I'm not going to write those broken-hearted, Oh-my-God-my-life-is-over sort of emotional songs because it's just not happened to me," she replies. "I prefer to write from experience rather than just write a whole load of trite stuff that doesn't make sense. If you think you're going to write a love song for the hell of it, then it's false. It doesn't mean anything to you, so how's it going to mean anything to the millions of people that might be listening to it?"

New songs and a second album will come when they come, she says, when there is time and space to be devoted to their creation. But time, the thing she most needs, is the one thing she can't buy. Still, even if Macdonald is weary of what she calls "the whole happy, happy dance" of promotional tours and endless interviews, the acclaim from her fans keeps coming - from feedback via the internet and from the crowds at that endless succession of live shows.

"That's the reason I'm doing this," she says. "I can write a song and it can make someone I've never met and who lives miles away from me feel good about themselves."

Her music does mean something to the millions of people listening to it, and that knowledge is what keeps pushing this bright young woman on - that and the fact she is still forging her own link in that ever-expanding chain of music.

Amy Macdonald plays the Hydro Connect festival, Inveraray, on Friday.