At her worst point, she was throwing up as many as seven times a day, which is like "putting a sledgehammer to your vocal cords," she tells PEOPLE in its new issue.
Finally, after successfully auditioning for Idol last fall, McPhee, 22, decided to seek help. "When I made it onto American Idol, I knew that food – my eating disorder – was the one thing really holding me back," she says. "I was bingeing my whole life away for days at a time … So when I got on the show, I said, 'You know what? I can do well in this competition. Let me give myself a chance and just get ahold of this thing.' "
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"I knew I had put off going to a treatment center long enough – I'd been struggling with bulimia since I was 17," she says. "Growing up in Los Angeles and spending all those years in dance class, I'd been conscious of body image at a young age, and I went through phases of exercising compulsively and starving myself. … Food was my crutch; it was how I dealt with emotions and uncomfortable situations."
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Although McPhee admits she's still learning to deal with stress – "I still bite my nails," she says – she's much happier these days. Of getting treatment for her illness, she says, "That's why I say American Idol saved my life, because if I hadn't auditioned I don't think I would have gotten a handle on food."
Pick up the new issue of PEOPLE, on newsstands Friday, for the full story.
For more on Katharine McPhee, visit TeenPeople.com.