That really sucks man. I know how you feel exactly and the pain will go away once you stop asking questions and stop trying to blame yourself or her for what happened. The best you can do is be around your friends and if you ever feel like you need to talk about something to them, do it, good friends will listen.

I dated a girl for four years and we broke up. The next year and a half was an up and down roller coaster for me. But time will set you straight. Good luck man, everything always works out for the best.