bullet444,
I can't think of too many situations (or any situations for that matter) where having your girlfriend cheat on you with her ex shows any signs of a lasting relationship. When this happens, it usually means that you aren't providing her with some relationship aspect that she craves, and her ex-boyfriend used to provide it better. This could be that the sex isn't as good between you two, or it could be that you're spending too much (or not enough) time together.
It can sometimes be hard to pinpoint where the actual problem is if you weren't already aware of it, but it also means the relationship is probably on a downward slope anyway. It's possible that her ex might have been white-knighting her and putting you down to try to win her attention back, but in general, if you're doing everything right, she wouldn't be spending time with her ex or listening to anything he has to say to begin with.
It would suit you much better to end the relationship entirely and start meeting new women. From there, you can start the relationship off the right way so that your girlfriend never even has thoughts about cheating on you.
- Franco