Dating Forum - I Don't Know How to Leave Him. Help!

We had been engaged for almost 4 years. I tried breaking up with him 4 or 5 times during the first year, but he always convinved me to give it another try. We had our son who is now two and a half and sharp as a tack. Over the past two years we've had our upa and downs such as any other relationship does, but it turned for the worst early 05. I was deployed for a month and was pulled aside my unit to be told that I had to go home due to something my doctor found in some tests every female takes prior to deployment. A week before I was scheduled to get on a plane and go home, I cal my fiance and tell him I should be leaving in about a week or so. In the same phone call, he tells me that he met some girl from another local town who's boyfriend had also recently deployed and that they had been hanging out quite a bit. Long story short, they had lunches, dinners, went to clubs...Things he and I never did anymore because we took care of our son who was at the time staying with my parents. The night I get home, I wake up and begin to wash my face in the sink. He walks around the corner and says, "by the way this is such-in-such'. The girl he told me about. I tried not to think anything of it, but she gave him a huge hug right in front of me, then turned and got in her car without a 'nice to see you home safe, or nice to meet you, or even thank you for letting me stay in your home...He insisted that nothing was going on. Later that week, I find out by him saying the wrong thing, that he had been to her apartment to help her prepare for deployment since her roommates had already moved out. He still sticks to his story. I end up returning to iraq, and two days after I got on the plane, i'm standing in front of my crying best friend, who happens to be a guy that i've never done anything with, and he tells me his wife has cheated on him. We talk for three hours before he tells me that one of the men she cheated on him with was my fiance. Here we are a year later, after trying to work things out. He has changed to be completely responsible and totally loves our son and I to death! Great guy, and certainly more respectful of our relationship. I can't get past it for the life of me, and the feelings I used to have for him are gone. I've tried leaving, we were trying a trial separation, but I just feel so guilty about hurting him, after he's tried so hard to grow up that I let him move out of the spare room, back into our bedroom. Our finances won't allow either of us to move out right now. It's so crazy! What do I do? Please help me! Should I stick to what my heart tells me and hope it doesn't effect my son too badly now, as it would later on? How do I get him to accept my indecisiveness and leave me alone?