Demi Lovato's Shocking Diagnosis

While in treatment, Lovato learned to alter her coping skills and found better ways to deal with her emotions.

"For the first time in my life, I started to feel," she said.

During the tough times in treatment, Lovato says it was the thought of her younger sister, Madison, who plays Juanita Solis, the daughter of Gabrielle on "Desperate Housewives," that got her through her darkest hours.

Heidi Gutman/ABC News

20/20 - Demi Lovato speaks to ABC News Anchor... View Full Size

Heidi Gutman/ABC News

20/20 - Demi Lovato speaks to ABC News Anchor Robin Roberts about her struggles with cutting and food addictions, and, for the first time, she talks about the depression that... View Full Caption

20/20 - Demi Lovato speaks to ABC News Anchor Robin Roberts about her struggles with cutting and food addictions, and, for the first time, she talks about the depression that has plagued her since childhood. A portion of the exclusive interview will air on GOOD MORNING AMERICA (7-9am, ET) and the interview will air on 20/20 on FRIDAY, APRIL 22 (10-11pm, ET) on the ABC Television Network. Close

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"A picture of my little sister, on my little bulletin board, was one of the main things that kept me going," she said. "I just kept thinking, 'OK, set this example for your little sister.'"

Lovato wanted to set an example -- not only for Madison, but for all the other women fighting the same issues.

"The real reason why I'm sitting down with you," she said, "is to open up the eyes of so many young girls, that it doesn't have to be this way."

Teen singer and actress Demi Lovato sat down with ABC News for her first television interview since leaving treatment for cutting and eating disorders. Watch Lovato share her story on the latest full episode of "20/20"