New Demetria photoshoot & article
When skinniness didn't provide her with the friends or happiness she yearned for, Demi sought solace in drugs. It was reported that she was snorting cocaine, but Demi - aware of the influence on her young fan base - is reluctant to go into detail.
Club promoters gave me drugs and alcohol in clubs
"It's something I don't really want to talk about," she says apologetically. "What I can say is that I was depressed. I would come off stage in front of 18,000 people and suddenly be alone in a hotel room. I'd come crashing down and would try to find a way to recreate that feeling, to stay 'up'."
Being based in Hollywood made it easier to get access to illegal substances.
"Promoters gave me drugs and alcohol in restaurants or clubs. They wanted me to come back so I would be seen there. They were basically kissing my ass," she says, a flash of anger briefly interrupting her sunny demeanour.
"I thought they were my friends. I thought I was having fun. Being a celebrity can be dangerous. Nobody says 'no'. That's why so many end up overdosing and dying. It could definitely have happened to me."
When the drugs didn't block out her pain, she cut herself. "It started with my wrists. People saw that, so I cut in places they couldn't see," she says.
"You do it because you feel so bad inside. You don't know how to take it out other than on yourself."
Demi - whose mother Dianna, a former cheerleader, divorced her father Patrick when Demi was just two - felt unable to confide in her parents.
"I'd just get scolded if they found out," she says. "They were worried, but I knew they wouldn't understand. I had a hard time opening up to friends. People were there for me but I didn't utilise them. Do I wish I had? Yes."
Instead, she threw herself into her work. Demi's first album, Don't Forget, was released in 2008. The following year she starred in the Disney film, Princess Protection Program, and released her second album, Here We Go Again. It shot to No.1 in the US charts.
"I went from movie to album to touring to television and back," she says. "Being in the limelight wasn't the root of my problems, but it didn't help. I never took more than two weeks off in four years and it caught up with me."
A long road
Nor does Demi plan to resume acting until she is fully confident. "I need to be secure in my body before I go back in front of the camera. Anyone in recovery from an eating disorder would find that triggering, and I'm not ready," she says.
She admits her stunning new size 10-12 body is taking some getting used to: "After so long being thin, it was terrifying being heavier. But I am a naturally curvy Hispanic girl. I don't deprive myself - I had a Kit Kat last night, but I don't eat s*** every day. I have a meal service that brings my food to my home so I don't have to think about being healthy."
Although she no longer drinks, Demi admits that she has self-harmed and made herself throw up since leaving rehab. "I've slipped up a few times, but each time I have learned from it, and it's become further apart," she says.
She's still BFFs with Miley Cyrus, and has found an unlikely ally in Cheryl Cole, who, after watching a recent MTV documentary about Demi's troubles, tweeted her support. "I was so excited. I'd love to meet her," Demi says. Currently single, her exes include Joe Jonas, 22, and actor Wilder Valderrama, 32.
"I'm not dating at all. I love having a boyfriend but need to be secure on my own first," she says.
Despite Demi having had a lifetime of troubles before she's even 20, she insists she has no regrets.
"There were times I wish I'd been a normal teenager so I could make mistakes and not be scrutinised. But I don't mourn the childhood I never had. I'd rather have been travelling the world and making albums than at high school."
And as for the bullies who made her life hell? "I don't think or care about them," she says. "I wouldn't change what I went through. I've learned from it and it's made me stronger."