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Britney Spears is adored by millions, but she's gone through periods of feeling very alone.

In a new E! documentary, "I Am Britney Jean," the pop star, 32, revealed that she's an introvert whom she believes is "not really made for this industry."

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"I have always been kind of shy, since I was a little girl. It's who I am to be modest, so I really can't help it," she said, according to the Huffington Post. To perform, "I turn into this different person… seriously, bipolar disorder."

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Spears also alluded to her 2007 public breakdown, after which she spent time in rehab. For the next few years, the mother of two tried to keep a low profile.

"There was a time in my life when I couldn't ever leave the house without 20 cars following me. I felt very alienated from the public," she said, later calling it a "dark period." "But as time passed, they lightened up and they kind of went away after I wouldn't come out of the house for like two years."