In this week’s PEOPLE cover story, Mariah Carey reveals for the first time her battle with bipolar disorder

In this week’s PEOPLE cover story, Mariah Carey reveals for the first time her battle with bipolar disorder.

Although she was first diagnosed in 2001 (when she was hospitalized for a physical and mental breakdown), “I didn’t want to believe it,” the superstar singer-songwriter tells PEOPLE editor in chief Jess Cagle.

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Carey says she finally sought treatment recently after “the hardest couple of years I’ve been through” — years of professional upheaval, an E! reality show and romantic drama.

“Until recently I lived in denial and isolation and in constant fear someone would expose me,” she says. “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.”

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One of the most successful singers of all time, with 18 No. 1 hits and more than 200 million records sold, Carey spent many of her years in the spotlight suffering in silence.

She is now in therapy and taking medication for bipolar II disorder, which involves periods of depression as well as hypomania (less severe than the mania associated with bipolar I disorder, but can still cause irritability, sleeplessness and hyperactivity).

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“I’m actually taking medication that seems to be pretty good. It’s not making me feel too tired or sluggish or anything like that. Finding the proper balance is what is most important,” Carey tells PEOPLE.

“For a long time I thought I had a severe sleep disorder,” continues Carey, now back in the studio working on an album due later this year. “But it wasn’t normal insomnia and I wasn’t lying awake counting sheep. 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.”

RELATED VIDEO: PEOPLE Exclusive: Mariah Carey Is Managing Her Bipolar Disorder With Medication & Therapy

Carey, who co-parents her 6-year-old twins Monroe and Moroccan with ex-husband Nick Cannon, says she decided to come forward because “I’m just in a really good place right now, where I’m comfortable discussing my struggles with bipolar II disorder. I’m hopeful we can get to a place where the stigma is lifted from people going through anything alone. It can be incredibly isolating. It does not have to define you and I refuse to allow it to define me or control me.”