The 90-minute film, "Britney: For the Record," airs Nov. 30 on MTV and the LOGO network. She is shown backstage at the MTV Video Music Awards; recording her new album, "Circus"; dodging the paparazzi; and goofing off with her father, Jamie, who controls her personal and financial affairs.
While her career takes a positive turn, Spears says she's sad and feels her life is "too in control."
"There's no excitement, there's no passion, there's no nothing. It's just like `Groundhog Day' every day, you know? So I'm really bored," she says, getting teary.
The mother of two young sons with ex-husband Kevin Federline says she "used to be a cool chick," but the paparazzi has made it impossible for her to be out and about. In the documentary, which was executive produced by Spears' manager Larry Rudolph, her security staff discusses hiring a decoy to fool the cameras.
Spears' turnaround kicked into high gear in September when she nabbed three VMAS, including video of the year. Now on the comeback trail, she has a hit song, "Womanizer," and is releasing "Circus" next month.
She talks about her two-year downward spiral, which included a divorce, a custody battle, public meltdowns and a sluggish performance at last year's VMAs, when she barely made it through a dance number of "Gimme More."
"I've been through a lot this year - well, actually the past two or three years," she says. "And my trust has really been battered. I've definitely grown up - big time. And I'm very weary of a lot of things. Very protective of myself."
She says sometimes she gets lonely.
"You're guarded," she says. "You have to be that way, otherwise you get taken advantage of and get in situations like I did the past year. I totally lost my way. I lost focus, I lost myself, I had that type of nature within me that wanted to rebel out.
"I never wanted to become one of those prisoner people - I always wanted to feel free and get in my car and go and not let people make me feel like I have to stay in my home. I think that was always the part of me that kinda got me in trouble," she says.
Spears dishes on Federline and ex-boyfriend Justin Timberlake in the film.
"With Justin, he was a part of the magnitude of what I'd become," says Spears, who ended a three-year relationship with Timberlake in 2002. "So then when he was gone, I was like, `What am I supposed to do with myself?' ... I would go out just to keep my mind busy, just to keep going."
While she was devastated over her breakup with Timberlake, she says she handled that better than her separation from Federline in November 2006.
"With Kevin, because I had two children with him, I did not know what to do with myself," she says. "I had built my dream home in Malibu, a huge house and a pool and a huge yard for the kids and I did everything for them. And just my world was that."
She says she married Federline for the wrong reasons instead of following her heart. As a result, she went down a "weird path."
She says her marriage disintegrated when Federline began concentrating on his rap career.
When it ended, "I felt so alone. I didn't really want to think about the reality of it. It was like, `I'm OK.' ... But I never really faced it, and I just ran," she says.