Renee Zellweger and Bradley Cooper's break up was one of Hollywood's most surprising splits this year--mostly because no one knew they were having any trouble. And while there have been rumblings of infidelity on the part of Cooper, an insider close to the former couple says that's not the case at all.

"If I had to pick any possible mistress it would be Brad's career," the source tells UsMagazine.com "He worked really hard to get into leading man status."

"Renee had to take a backseat," the insider continued. "She traveled with him to make it work."

At first, it appeared Zellweger had no problem putting her own career on hold as well as moving to Los Angeles to make things work. "She's accomplished a lot of her professional goals," the source revealed. "So Renee took some time out to be a great girlfriend and see if this is what it took to make a relationship work."