Justin Timberlake and Britney Spears by Denise Truscello/WireImage.com
bailed at the last minute on mixing a duet with red-hot former flame
, robbing herself of a
showstopping opener for the upcoming VMAs
, who was producing the unlikely duet, "set aside a week out of his crazy schedule to do this and then, just before she was supposed to fly out, Britney abruptly canceled," a source tells the
New York Post
's Page Six. "It's crazy. She's looking for a comeback, and this would have not only been a huge hit, but something [that would have] really blown everyone away."
Just how crazy
Brit? (Alleged hookups with
aside?) "She needs help," another source tells the
. "It's sad because what she's got - we've heard it's like bipolar disorder - can easily be treated with medication, but she won't do it."