i like what awfranklin and jenn said.
Just to add to that, and some of this you might already know:
1) Call her calmly and end it quickly. Don't have a long conversation. Don't take her phone calls afterward. Not again for another week at least. This is the toughest part. And then...
2) What helped me the most was to focus on myself. Instead of eating a tub of ice cream like in the movies... work out harder, cut any bad drug/eating habits, listen to music that you love. This will help you get through it in many ways. Before I knew it I felt great and ready to move on.
2) Remember you're above cheating (I'm assuming). There are girls out there that won't cheat--and especially don't become the jealous/snoopy type with your next girl
3) Hang out with your most trustworthy guy friends for awhile. They'll help ya out. They'll probably call her a bitch and tell you that you deserve better. Which you do. You'll see them succeeding at being single or dating, and you'll be ready to follow their lead
4) Time is your best friend. It will heal your wound and provide other opportunities for you.