In a People magazine article due on newsstands Friday, the singer says she didn't believe it when she was first diagnosed after she was hospitalized for a physical and mental breakdown in 2001.
The 48-year-old says she lived in denial and isolation and feared someone would expose her.
Carey is in therapy and taking medication for bipolar II, which involves periods of depression and hypomania, which involves irritability.
Carey says she's taking medication that's not making her feel too tired or sluggish.
"I got back to doing what I love," she says. "Writing songs and making music."