Well I won't speak to weather or not you should marry since I don't know you.
Outside of the things others have mentioned (stop cohabiting, remove the sexual component, etc) one thing that struck me was when you said:
Originally Posted by sanderson1984
... I dont practice my faith as often as I should ...
I would first suggest getting to Mass and praying. This could help calm some of the inner turmoil. Obviously your current living arraignments preclude partaking of the Eucharist but placing a focus on you spiritual life might help you with the graces you need to provide some stability. If your fiance is not regularly nourishing his spiritual life I would suggest the same for him.
As for the sexual infidelity being brought up time and time again. It sound like your fiance still hasn't fully dealt with it. I know when I was younger that I was very insecure and often questioned if I was good enough for my girl friend (now my wife). If she had cheated on me that would have just reinforced how undeserving I was in my mind. It is possible that you boy friend feels the same way. You and he very likely need professional help to get past that. That is one of those things that can haunt a relationship so saying he forgives you in the calm between storms might not be enough. He may want to forgive and forget with every ounce of his being, but without help desire is not always enough. Here is another place where prayer might help him.
Regardless of what you guys do it sounds like you both need to learn conflict management skills. Every couple fights on occasion, but frequent fights where things are said that are calculated to hurt the other person is not a healthy form of communication. This is another place where couple counseling or a conflict skills class could really help.