It is not your fault. There is nothing you could have done to keep him from cheating or to save your marriage. It takes two people to make a marriage work. And he quite simply wasn't interested. His cheating is not about you, but about something missing in him. You cannot control another person's actions or feelings. You cannot change the past. As much as you'd like to, no amount of reliving those memories will change anything.
Do not blame yourself. This is hard, because you assume that there must be something wrong with you if he can find happiness with someone else. But that happiness is an illusion. You don't need to concern yourself with them or what they are doing or why they did it. You can't make sense out of nonsense. It will make you crazy. Focus on yourself and how you can come through this a stronger, wiser person. It won't always hurt this much. The pain is temporary, so don't give up. If you hang in there, it'll get better.
Be good to yourself and love yourself. Lean on God, your friends, your family. There are still a lot of people who love you and will be there for you if you let them. Most importantly, forgive yourself. You will come up with a huge list of things that if you'd just done x,y, or z, this wouldn't have happened. But, you have to realize that no one knows the future and you could have done all those things and more and still been here. So be good to yourself, let go of him, let go of the past. You deserve so much more than this. There are better days ahead.