You re my hero dude! Ever since I joined sosuave i ve been monitoring these "girlfriend did X... What should I do?" threads. 9/10 of the guys asking for advice end up not following the advice, either staying with the girl when they shouldn't, begging her, giving her a second chance, etc...
for 2 years I ve been waiting to find somebody like you, who would just dump the cheating biatch right then and there.
Everything else has already been said pretty much, i d just emphasize the importance of no contact. Time heals all wounds, and by that I mean time away from her. But contacting her, you keep resetting your clock, so don t do it.
Also, you should not feel bad AT ALL for what you did to her. She f.ucked up big time and she s now paying the consequences. This was NOT a mistake! This was a girl living with her boyfriend, consciously making plans to spend a few nights a week with another guy, getting pounded. This involves constant deception, constant lying to your face, constant inventing stories to cover up her cheating ways. Where do you see a mistake? It was all planned out, her only mistake was that she got caught by you. Now she won't be able to play this game, which she enjoyed, any longer. And that is the reason why she feels so bad. That is the mistake that really hurts her the most, not the fact that she crushed your heart.
Bottom line: if this girl cared about you as much as you cared about her, she would have not put herself in this situation.
Forget about her and move on