I'm not exactly giving this as advice to you, but for ME, I just went and kissed other people myself. Made me feel a lot better and then with time I didn't care much anymore!
It does hurt, my ex cheated on my with HIS ex. I remember the thoughts at the time being painful, knowing him and her were together. But at the same time, I made myself think "well, that's life. We weren't going to exactly end up married, it was rocky and ended for a reason, I don't like the thoughts but what am I going to do?"
I also thought that he got with other people before me, so what's the difference.
I really think your answer, though, is to get out the house, get busy, and perhaps even go get involved with some others girls…even if it's just by starting to flirt or even just TALK to girls again with a couple mates in a club - doesn't have to be flirty. It does make you feel better to stay active and busy. Don't stick yourself at home with only your thoughts.