The 21-year-old singer said that Lovato is the one person with whom she could share her "darkest secrets", reported E! Online.
"Demi's very, very strong. She's the only person that I would tell the deepest, darkest secrets to. The only person," she said.
"I've known her since we were 7, and we did fall apart for a while," she added.
Lovato sought treatment for an eating disorder and self-harm issues.
"She was going through things and I was so young and it was confusing. I processed it saying, 'OK, I don't understand what she's going through so I'm just going to do this.' I don't think it was fair, and I'm so happy that I have her back in my life now," she said.