Mariah Carey has revealed that she struggles with bipolar II disorder.

The singer, who infamously had a public breakdown in 2001 on "TRL," is opening up about her mental illness for the first time in People magazine.

It was that year, when she was hospitalized for a mental breakdown, that she was first diagnosed with bipolar disorder.

“I didn’t want to believe it,” Carey said.

Carey said that for a while she thought she had a sleep disorder, but now realizes she was having manic and depressive episodes.

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“For a long time I thought I had a severe sleep disorder, but it wasn’t normal insomnia and I wasn’t lying awake counting sheep,” Carey said. “I was working and working and working … I was irritable and in constant fear of letting people down. It turns out that I was experiencing a form of mania. Eventually I would just hit a wall. I guess my depressive episodes were characterized by having very low energy. I would feel so lonely and sad — even guilty that I wasn’t doing what I needed to be doing for my career.”

According to the National Institute of Mental Health, bipolar II disorder is characterized by "a pattern of depressive episodes and hypomanic episodes," but short of manic symptoms that require immediate hospital care. Some symptoms of a manic episode include increased activity levels, agitation or irritability and trouble sleeping. NIMH says some symptoms of a depressive episode include having trouble sleeping or sleeping too much, forgetting things a lot, eating too much or too little, having little energy, and thoughts of death or suicide.

Ultimately, the 48-year-old mother of two got help, and is now in therapy. She is also taking medication, which she said “seems to be pretty good” and doesn’t make her feel too tired or sluggish.

Carey is hopeful that revealing her battle with bipolar II disorder will help decrease the stigma associated with mental health.

“Until recently I lived in denial and isolation and in constant fear someone would expose me,” Carey said. “It was too heavy a burden to carry and I simply couldn’t do that anymore. I sought and received treatment, I put positive people around me and I got back to doing what I love — writing songs and making music.”