Who is to blame for my unhealthy marriage?

You shouldn't look for who's to blame, but figure out a way to fix this problem. It was not a good idea to get angry right off the bat and then cheat on your wife in such a way. Think about how the reason could have made your x-girlfriend feel for one, and second, what if you sleeping w/ that girlfriend caused your wife to decide to cheat on you? Marriage is supposed to be a partnership, and your wife need to calmly discuss what you feel and DON'T start playing the 'blame game'. That will get you no where. A calm conversation can go a long way to a more satisfying ending. Think about your daughter and what's best for her. A secure family and loving parents. You can bring this up with your wife and ask what you two could do to make this better, and you can tell her you're sorry for jumping to conclusions and sleeping w/ your x-girlfriend and ask she stop sleeping around with the other man. Your family could remain friends with him and his daughter, but the cheating NEEDS TO STOP. I hope this helps! :)