Glad to hear you are feeling a bit better about this. I too found out that my girlfriend had cheated on me after two and a half years of incredible closeness, trust and love. The hardest part for me was the fact that she lied about it. But, as others have said, this is the cold, hard truth, and it does help you to move on:
People (and I have been a cheater in the past and know this to be true) only cheat when they are unhappy in their relationship. Which is awful for you, but it means that you were heading for the rocks, and such a shocking end at least means a real end, and not a long, lingering "will we make it, won't we make it" scenario. However hard it is, you have to accept that you were wrong for each other and that spending any longer in that relationship would not have been good for you. And never blame yourself, which can be hard (believe me I know). We all make hundreds of mistakes in any relationship but those mistakes don't matter when you are in a mutually loving relationship. If they do matter enough so that the other person cheats, it is a clear indication that your personalities were not good matches.
So, as shallow as it sounds, keep your chin up and move forwards.