I agree with Beef. Girls will come and girls will go. Holding on to the past will hurt your dating life. ( i guarantee you that you hvae not been doing well in dating game b/c u are hurting your self thinking about her).
We have all been through it. I have been through it as well.
If i were you i would actually freaking go out and just chill with them. Dress nicely. Make your ex laugh, make her friends laugh. Sit back relax. Make an enviornment in which you can tell that your ex and her friend(s) notice that you are the freaking man. I am not saying that make your ex jealous. But you gonna have to make your self believe that she is not the one and you can find better. My first love cheated on me quite a bit of times and it used hurt but after reading Double your dating, tips from other dating web sites, (here is a good one:
http://www.fastseduction.com/mailbag/?top), going out with the guys who knew what to do around girls and could pick them up and set a date or sleep with them in a hear beat i realized that I have such a potential to find me even better. Today i am dating a girl who everyone wants to be with she is 10/10, she is on a our college dance team ( 16 out of 60 made the team) and i am like 7.8/10. All you have to do is sit back, relax, enjoy your self, and most of all have a lot of freaking confidence. and most of all don t hurt your self thinking about her all the time you will screw your self over.