That sucks man.
It happens to the majority of people at least once in their life, though, so you're not alone that's for sure (even on this forum we've had multiple people make threads like this).
The very best thing to do is to keep perspective. This is what humans do, it's part of who we are as a race. Humans were not meant to be with 1 person all our life, there's a reason why the vast majority of people cheat or have been cheated on at least once in their life, and the ones who claim they haven't probably have not realized that they have been cheated on, they are just unaware. It does not help you emotionally, but it helps you keep perspective that you are not alone, it happens to the vast majority, as that's what humans do.
Second of all, life is WAY too short to get caught up in emotions like this. Again, you can't help it, but to keep perspective - You need to either move on or take her back, if you choose to move on as you seem to have, then do exactly that - move on. You will still feel bad, but do not let it halt your life for a second.
Third, look at it as a blessing. If someone cheats on you, then that's that. They are cheaters, they have broken the "law" of relationships and clearly it's a major issue and a relationship breaking one. You will still live for a long time my friend, move on and remember that 1 girl can't and shouldn't control your life. There's much, MUCH more in this world to look at than another person.
Just try to relax, be calm, and realize everything you do daily you can still do, life goes on, and you will find someone else.
"You lack the requisite spine and testicular fortitude to study under me"
"You do not know the first note of the music that moves me."