Britney Spears has opened up about her much-publicised breakdown in a revealing documentary set to hit U.S. TV screens next month.

The pop princess talks candidly about her marriage breakdown, isolation and losing custody of her sons Sean Preston and Jayden James.

Britney tells MTV, "I sit there and I'll look back and I'm like: I'm a smart person. What the hell was I thinking?

"I've been through a lot in the past two or three years, and there's a lot that people don't know.

She adds, "Sometimes I think I get kind of lonely because you don't open the gate up that much, you know what I mean?

"You're guarded. You have to be that way, so I'm kind of stuck in this place and it's like: How do you deal? And you just cope, and that's what I do. I just cope with it, every day."

For the Record will air in the U.S. on November 20.