Britney Spears admits her marriage to Kevin Federline was a mistake.

In her new 90-minute MTV documentary, “Britney: For the Record,” the pop star talks candidly about her three-year marriage to Federline.

“I think I married for the wrong reasons,” Spears said. “Instead of following my heart and doing what made me really happy, I just did it for the sake of the idea of everything.”

 The clip of the new show, scheduled to air on MTV on Nov. 30, was posted on Sky1.com, a site for a British TV channel.

In the clip, as Spears talks about Federline, her single “Womanizer” plays in the background.

“Your music is a reflection of what you’re going through. It’s such a part of me, the record, because of what I’ve gone through,” she added.

In additional excerpts from the documentary posted by U.K.’s The Sun, Spears admitted to struggling with the restraints put on her life over the past year.

“There’s no excitement, there’s no passion. I have really good days, and then I have bad days,” Spears said.

She even went as far as to say living under the control of others (her father maintains his conservatorship) is worse than being locked up.

“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,” Spears continued. “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.”

Britney Spears and Kevin Federline’s custody dispute looks set to head to trial in August after the couple failed to reach agreement in mediation, it was reported on Friday.

Federline’s lawyer Mark Vincent Kaplan told the E! Online website that although day-long talks between lawyers for the two sides were positive, no settlement was reached and a trial was likely on August 25.

“The mediation didn’t result in an agreement that would avoid the trial … The mood was comfortable, positive and it was a mood that was consistent with opening at least a great dialogue, which is necessary,” Kaplan said.

Kaplan said Federline, 30, wants to retain sole legal and physical custody of the couple’s two young children, Sean Preston and Jayden James, while allowing Spears, 26, visitation rights.

“Kevin is seeking to maintain the sole legal and physical custody that he presently has,” Kaplan said. “Kevin has had sole legal custody since January. He wants that to become the permanent order at trial, and the visitation to be consistent with what the court expanded it to this week.”

A court this week granted Spears overnight visitation with the children in addition to two day visits per week.

Spears’ visitation rights were severely restricted earlier this year amid spiraling personal problems and fears for the singer’s mental health.

However, the pop star has appeared to have been getting her life back on track in recent months, after she was twice rushed to hospital earlier this year for treatment in a psychiatric unit.

Spears, one of the most successful pop stars of her generation, has been locked in the custody saga with Federline since the couple split in 2006.

Her access to her children was restricted last October after she failed to submit to random drug testing as demanded at an earlier hearing where the court ruled she was a “habitual and continuous drug user.”

Spears shot to superstardom in late 1998 with her smash-hit debut album “Baby One More Time,” which she followed with another chart-topping success the following year, “Oops! … I Did It Again.”

Kevin Federline will “always love” his ex-wife Britney Spears and has promised to always “be there” when she needs him.

Federline, who has full custody of the former couple’s two sons, insists Spears will always be important to him as she is the mother of his children, Sean Preston, 2, and Jayden James, 18 months.

The dancer/rapper also lavishes praise on his ex-girlfriend, Shar Jackson, who is mother to his two other children, Kori Madison, 6, and Kaleb Michael Jackson, 4.

He tells In Touch Weekly, “I will always love the mothers of my children, and I will always be there for both of them. I believe that any good energy I put out there brings more good energy into my own life. The relationships with my kids and family will last my whole life and are the most important to me.”

And Federline is determined that his children with Spears will grow up in a happy, stable environment.

He says, “Their needs define my schedule every day. It is so very important to me that my children will always know and never question, that no matter what happens, they have a mummy and daddy who love them very much.”

Federline was granted full physical custody of the boys last year after Spears failed to adhere to court-ordered medical tests. The troubled singer was also stripped of her visitation rights to her kids in January, but Federline and Spears have since agreed to a new deal that lets Spears have limited access to her children.

No first-day jitters for Britney Spears. Arriving promptly at 7:30 a.m. Wednesday on the L.A. lot of the CBS sitcom How I Met Your Mother with her assistant Brett in tow, Spears quickly got to work impressing her costars.

“It pains me to report that on the first day of the shoot, Britney knew her lines better than I knew mine,”  “She’s been great to work with.”

n the episode, which will air March 24, Spears plays a receptionist at a dermatologist’s office, who falls for one of the show’s main characters, Ted, played by Radnor. She is expected back on set tomorrow for a second day of shooting.

Spears first met the cast on Monday during a table read. A set source told PEOPLE, “She was a bit nervous, but didn’t let it get to her… She really seemed to enjoy herself and feel comfortable.”

Lawyers around troubled singer Britney Spears battled in court on Monday over her parenting issues and $1 million in legal bills with one judge ruling the pop star should receive $1,500 a week to spend.

That sum must seem like a small amount for the former child star, who in 2002 was named the world’s No. 1 celebrity by Forbes magazine after earning $39.2 million in one year.

But since 2006, Spears’ life has spiraled downward. She divorced husband Kevin Federline, spent time in rehab, lost custody of her and Federline’s sons, and this year was hospitalized twice for psychiatric evaluation.

In January, her father Jamie Spears won control of her personal and business affairs via a conservatorship, and on Monday Court Commissioner Reva Goetz said he could give Britney a debit or credit card with the $1,500 per week to spend.

Goetz also authorized a $135,000 bank account, controlled by Jamie Spears and a second co-conservator, to pay bills and said conservators could hire an entertainment attorney.

Spears, 26, would appear to need some legal advice because she will appear as a guest star on a new episode of CBS comedy “How I Met Your Mother,” a network spokeswoman said.

A second source close to the show said the episode would tape this week and likely air on March 24, adding that Spears took part in a “table read” rehearsal on Monday.

As a result, Spears was not in court again after missing many hearings throughout 2007 and this year. Her court-appointed attorney Samuel Ingham said he did not know why she was not coming.