I have had two experiences with rebound relationships. Both happened 3 months after I broke up. And both relationships lasted for 3 years.
The trouble with a rebound relationship is although you are with the new person, it's hard to forget the previous one. And this somehow will affect the new one.
But let me tell you this. No matter how you still love your ex, it's best not to get back together especially now that she is with someone else. One of the reasons is because even if you get back together her past with her new lover will haunt you.
It's best to give yourself time to get over the last relationship, start anew and meet someone else. It's a good feeling and liberating to fall in love all over again.