The singer, 41, reflects on her relationship with Justin, 42, in her bombshell memoir, The Woman In Me.
Britney and Justin dated from 1999-2002 after they both became child stars when they starred in the Mickey Mouse Club back in the early nineties.
In an excerpt obtained by TIME, Britney said Justin moved on fast and addressed her short-lived fling with Colin Farrell in the wake of their split.
Britney said she was devastated after Justin bedded ‘six or seven girls’ in the weeks after they broke up.
She said: ‘He was a girl’s dream. I was in love with him.’ After Justin’s relationship with Britney, he embarked on a romance with Cameron Diaz.
They met a year after his split from Britney when she presented him with Best Burp at the 2003 Kids Choice Awards.
She was 31 while he was 22 at the time. They went on to date for nearly four years.
Britney said she wasn’t over her ex-boyfriend when she met Colin, 47, through a ‘club promoter friend’ before visiting him on the set of his 2003 movie S.W.A.T.
They had a short-lived fling which she said was passionate and likened it to a street fight.
‘As I had before when I’d felt too attached to a man, I tried to convince myself in every way that it was not a big deal, that we were just having fun, that in this case, I was vulnerable because I wasn’t over Justin yet,’ Britney wrote.