Britney Spears' comeback is bittersweet - because the star feels "no excitement, no passion" for singing after her infamous breakdown.

The star was hospitalised twice earlier this year and lost custody of her sons Sean Preston, three, and two-year-old Jayden James after a series of public meltdowns, including a headline-grabbing incident when she shaved her head.

Speaking in new MTV documentary, Britney: For The Record, the singer says, "I have really good days, and then I have bad days. Even when you go to jail you know there's the time when you're gonna get out.

"But in this situation, it's never ending. It's just like Groundhog Day every day."

The star's fortune is being kept under indefinite control by father Jamie as part of a co-conservatorship agreement with the courts - but Spears is wary that those closest to her are stifling her creativity.

She continues, "I think it's too in control. If I wasn't under the restraints I'm under, I'd feel so liberated. When I tell them the way I feel, it's like they hear but they're really not listening."

And Spears, who is enjoying success in the U.S. and U.K. with new single Womanizer, is convinced she is still paying for the breakdown she suffered in the public eye.