At last, a "revelation" about Britney Spears that does not involve dodging paparazzi, shaving her head, or having her private life picked over in court: after a nightmare year, American's most volatile young pop star may very well have rediscovered her marbles.

That, at least, is the shock conclusion being drawn by anyone lucky enough to have had a sneak preview of a new TV documentary in which the 26-year-old singer talks freely about her career, romantic history, and the mental illness that saw her sectioned by a judge earlier this year.

Spears, who last night made a special appearance on the ITV talent show The X Factor as part of a European visit to promote her new CD Circus, has rarely agreed to in-depth TV interviews. But she was followed for more than three months by the makers of Britney: For the Record.

The 90-minute show hits America's screens tonight before being broadcast on Sky One tomorrow and then in almost every country in which people have avidly followed Spears's progress since she flounced into the public consciousness wearing a schoolgirl's uniform almost 10 years ago.

In lifting the lid on her life of luxury hotels, badly lit rehearsal rooms and cat-and-mouse car chases with the dozens of photographers who still follow her every move, Spears manages to portray herself as bright and articulate, but rather lonely.

"I miss going out and doing stuff, or seeing a guy and hanging out, the way I used to live," she says. "I was a pretty cool chick. I'm not really that way any more... Sometimes it can get lonely... So I'm just stuck in this place... I just cope, and that's what I do, every day."

She will trace the public disintegration that saw her sectioned and placed under the guardianship of her father, James, back to the break-up of her three-year marriage to Kevin Federline, the father of her two children, in 2007.

"I think I married for the wrong reasons. Instead of following my heart and doing what really made me happy, I just did it for the idea of everything. And when it ended, I felt so alone... Now, I sit, and I look back, and I'm like 'what the hell was I thinking?'"

Spears is now firmly on the comeback trail, and also reveals a whiff of discontent about the legal rulings that give her only limited time with her sons Jayden and Sean Preston, and have put her father and lawyer in control of her assets and lifestyle.

"I think it's too in control. If I wasn't under the restraints I'm under right now... with all the lawyers and doctors and people analysing me every day... I'd feel so liberated."

There has been some scepticism about whether the documentary's stated "warts-and-all" remit has been fulfilled. Spears's manager, Larry Rudolph, is listed as one of the show's executive producers, and her father is thought to have maintained rights to "final cut" over the programme.

The rise, fall, and rise again

1993 The 11-year-old is discovered on 'Mickey Mouse Club' TV show.

1998 Debut single 'Baby One More Time'.

2000 Success for album 'Oops!... I Did it Again'.

2003 Kisses Madonna at the MTV Awards.

2004 Marries Jason Allen Alexander in Las Vegas. It lasts 55 hours.

2004 Marries Kevin Federline.

2005-06 Gives birth to her two sons a year apart.

2006 Files for divorce.

Feb 2007 Causes a stir by shaving her head.

Sept 2007 Comeback appearance at the MTV awards unsuccessful.

Oct 2007 Loses custody of her two children. Faces charges of hit and run and driving without a licence.

Sept 2008 Comeback with release of single 'Womanizer'.