I don't know if it counts because it was a less serious relationship, but I think I understand the basics on how you must feel. My girlfriend of 2 years cheated on me with a married guy twice her/my age. I was 18 at the time and she was 17. I've been wasting a whole freaking year getting drunk, going to sleep, getting up, regretting what I have done and getting drunk again to get over that shit. When I finally got a place to study, man I think it saved my life. Turned around 180 degrees.
The one thing I learned is don't start regretting the time you spent together, it won't make you feel any better. Don't blame yourself for whatever reason. Don't regret anything, when I look back at that time I wouldn't change a single thing I've done. Life is a series of decisions and since then I've started to live it like I mean it. I might screw up (a lot actually), but I mean every thing I do. No forgiveness needed.
Don't know if that helps at all.
Now that I went all philosophical on this thread I'll get some coffee.