Mike.....you are on point here and very good advice.
Another point that needs to be made, people do not cheat on you because of you, they cheat because of themselves....it's their choice, their free will, their character. Now maybe there were issues and this other guy looked better for a minute but that doesn't make you any worse or better....it just is what it is and it is reflective on her, not you.
I am not saying that you aren't to blame for some of the problems in your relationship or the end of it, obviously it takes two people to make or break any relationship and you did play a roll in the demise of this particular relationship but only 50%. I advise you to do as Mike suggested and after some time take a step away and look at this from the outside in and try to figure out where and how this fell apart and what your part was in that and then focus on changing that.......work on becoming again the person she originally fell in love with.
Forgive her, and yourself and whatever you do don't hate on the other guy......it will get you no where fast and make them stronger together.
Completely ignore the advice of Iloverainbows, sorry but it's just way off the mark and it will just make things much worse for you and her.
Good luck, not that you need it. This is new (the breakup) if you handle yourself with dignity and respect and be kind (not a doormat, just not mean) and give her forgiveness and space......I bet it will turn around before you know it. Just stay positive.