My current girlfriend cheated on her ex-boyfriend with me, which I found out a few months into the relationship. I later asked her what caused her to do so, and she said something along the lines of, "I just didn't like him anymore. We had been dating for a couple years and he just kept pushing on getting married and having kids, which I didn't want at the time. But I stayed in it because it was better than being single, and he was otherwise a pretty nice guy. But then I met [Pintergeist] and you're just totally different from him." She did emphasize to me that she felt guilty about the whole situation, but I'm not sure if she ever made amends with him.
I've never cheated, nor have I ever been cheated on, so I can only imagine the emotions that come with being betrayed. But the way I look at it is that my girlfriend cheated on her ex because I seem to offer her with some physical, emotional or psychological value that he couldn't. I have no idea who he is or what he's like, but for some reason she finds me different in enough ways that she'd be willing to date me instead. I would say that's a reasonable answer as to why people would consider cheating.
If she were to cheat on me, I simply have to concede that the other person was able to satisfy her needs and desires better than I could. I'm sure it won't make it any less painful at the heat of the moment, but it's just reality. It's her choice, and I can't force her to stay. But when my girlfriend and I were having that previous conversation, we agreed that if were to find someone better that we would just tell each other.
Try to look the situation objectively. For one, it could've dragged on for much longer. If it hurts now, imagine it happening a few years down the road. I promise these feelings will pass in due time. And don't bother blaming yourself, her, or the other guy because it won't do much of anything. Instead, treat it as a learning experience. When sort of signs did you notice? Was the relationship interesting and mutually beneficial? Why did the relationship not work? Answering these questions will only help to improve your interpersonal skills and future relationships.
Good luck, sir. I'm sorry your relationship had to end that way.