Lisa Milne, one of the finest operatic voices Scotland has ever produced, has announced she will be quitting the main stage and devoting the rest of her career to teaching.

"I felt I had come to the end of my life as an artist," Milne says, typically frank during the first detailed interview she has given since making the decision. In October she takes up an official position in the vocal department at the Royal Conservatoire of Scotland, her own alma mater and an institution that says it is "over the moon" to welcome the world-renown soprano onto its faculty.

Milne explains that she made the decision "due to loads of factors - they all accumulated". She's talking from her home in Aberdeen; she's just been out walking her dogs, she tells me, and the east-coast sun is high in the sky. She sounds breezy and self-assured, and laughs heartily when I suggest as much.

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"Well, that's good to hear! I've not had the easiest time of it. There have been four deaths in my family in recent years and I've had some ill health of my own. There was some idiot out there writing defamatory statements about me and refusing to withdraw them until we threatened legal action. I don't understand why someone I have never met was determined to wreak so much havoc on my life - and succeeded, too, because dealing with it was massively exhausting and stressful."

She acknowledges that her performance diary had seen a drop in future engagements, "probably because I had a reputation for cancelling". There is no question that her voice, so ardent, so supple and emotive, such a powerful musical storyteller, would have long been welcomed on stages around the world. But in recent years Milne was unable to commit to the long periods required for opera rehearsals and performances because of difficulties in her personal life.

"Last year a critic wrote - quite carelessly and quite hurtfully - that anyone booking me nowadays would need to have a standby up their sleeve. Sure, I've had to cancel engagements, but he had no idea of my family circumstances. When I look in the mirror, I don't feel any guilt for the choices I've made. Should I feel bad that I prioritised caring for my dad when he was heartbroken after my mum died? Nah. I'm happy to live with that."

Milne lists other contributing factors behind her decision to withdraw from the operatic front line. The atmosphere in the singing profession has "hugely changed", she says, since she first came to international attention almost 20 years ago. "Things have become very corporate. My agents in London used to have three little rooms; now they're a huge open-plan office with hundreds of staff. I didn't feel there was much integrity in what I was doing for them."

And she describes her struggle to comply with "industry image standards" as "wearying and demoralising". She speaks openly about undergoing a gastric sleeve operation and breast reduction "in order to look right for the stage", and says she became fed up of "constantly being told I wasn't pretty enough, wasn't slim enough, wasn't tall enough. Eventually I thought, 's*** to this'. At some point I just want to be me."

When I last interviewed Milne, she described singing as the "anchor" that helped her through the period around her mother's death. "In some ways, Mum not being around has actually given me a new strength," she said just over a year ago. "Near the end she told me, 'don't you dare stop singing'. She would be furious if I stopped."

Now Milne confides that the stress of balancing professional commitments with a tough set of personal circumstances was becoming too corrosive. "Performing internationally can be a lonely old life, especially when you're away from home worrying about your loved ones. Just weeks after Mum died I was singing Don Giovanni at Sottish Opera. It meant I lost touch with the grief that the rest of my family was going through at home, and it took until this year for it to catch up on me."

Meanwhile the one area of work that Milne continued to enjoy was teaching. "I'd come back from giving masterclasses at the RCS and my fiancé would say to me, 'You know, you always return from teaching with this glow about you?' Gradually I realised that the best use of my energy and musical gifts is helping students. Not much can beat the excitement I get from watching young artists start to flourish."

She recounts the day she "bit the bullet" and called her agents to announce her decision. "I said, 'Look, there's no work in my diary and I know my reliability is problem.' It was the agency I'd been with since my early twenties. It was like getting divorced. They suggested I tried another company but I couldn't fathom that. Loads of people in the profession told me I was mad, but I don't have any regrets. The fact that people seem to respect me as an artist is emboldening. It's given me the self-confidence I'll need to be a good teacher."

It is hard to imagine that the ebullient, effusive Milne will be anything less than inspirational as a pedagogue, but she says she is "bloody terrified" of the challenge. Work with students one-to-one and in masterclasses, she says what she most wants to convey to young singers is the 'x-factor'.

"It's something classical music tries hard to avoid, but it's crucial. Technique is all well and good, but it isn't what makes you stand out. I want students to make me sit up and listen. I'll make them sing things over and over until I literally have goosebumps. I tell girls to imagine they're singing to some hunk with his top off..." She lets out a cackle, hearty and infectious. "That's the kind of charisma they need to produce! And it needs to happens before they even start to sing, from the minute they walk on stage. It's about commanding a room." Sometimes it helps to have a specific idol in your head performing, she says. Hers was always Shirley Bassey.

Milne doesn't intend to stop singing altogether. She "might be persuaded" to give the odd recital at the RCS, and plans to write comedy skits with her "old sparring partner" Joyce Falconer. Then there's the Jacques Brel songbook she loves so well, and when we last spoke she described another side to her musical personality that, she said then, rarely gets an outing.

"My heart and soul is in the Delta blues," she told me, "but I can't sing it too much because that would seriously damage my Mozart voice. Maybe when I get too old for the classical stuff I'll call myself Big Momma Milne and tour the Highlands with a wee backing combo."

A year ago I replied that she should probably hold that thought because we wanted to hear plenty more of her Mozart voice. Now she just shrugs.

"I've come to a crossroads, to use a nice blues image, and I wasn't happy in the direction I'd been going. So watch this space for the blues band."