No one was more surprised to learn that Demi Lovato is bipolar than the singer herself.
"I had no idea that I was even bipolar until I went into treatment," the Disney star reveals on 20/20 (airing April 22 at 10 p.m. EST). "I was actually manic a lot of the times that I would take on workloads, and I would say, 'Yes, I can do this, I can do this, I can do this.' I was conquering the world, but then I would come crashing down, and I would be more depressed than ever."
Teased for her weight as a kid, Lovato developed an eating disorder that turned into a lifelong struggle with food. "That's kind of what I've been dealing with ever since," she admits. "I was compulsively overeating when I was eight years old. So, I guess, for the past 10 years, I've had a really unhealthy relationship with food."
"It was a way of expressing my own shame, of myself, on my own body," the Princess Protection Program star says. "I was matching the inside to the outside. And there were some times where my emotions were just so built up, I didn't know what to do. The only way that I could get instant gratification was through an immediate release on myself."
The dangerous coping mechanism continued throughout Lovato's teen years. When she began touring with the Jonas Brothers in the summer of 2010 -- alongside her ex, Joe Jonas, no less -- Lovato once again using food as a means of control.
"I was performing concerts on an empty stomach," she said. "I was losing my voice from purging. I was self-medicating. I was not taking medication for depression, and I literally was so emotionally whacked out that I took it out on someone that meant a lot to me."
During the South American leg of the Camp Rock 2 tour, Lovato physically struck one of her backup dancers, Alex Welch. "I take 100 percent full responsibility," Lovato now says. "I feel horrible. She was my friend."
The singer's family intervened and suggested treatment. "They sat me down and said, 'You can't live like this,'" Lovato recalls.
The "Here I Go Again" singer immediately quit the tour and checked into Timberline Knolls -- a residential treatment center in Illinois -- where she found healthier ways to deal with her emotions.
"A picture of my little sister on my little bulletin board was one of the main things that kept me going," she recalls. "I just kept thinking, 'OK, set this example for your little sister.'"
Though Lovato recently announced she will not be returning to the Disney Channel series Sonny With a Chance, the teen star says she hopes to become an advocate for girls facing similar struggles.
"The real reason why I'm sitting down with you," Lovato told ABC News' Robin Roberts, "is to open up the eyes of so many young girls, that it doesn't have to be this way."