How do you know when to stay in a relationship or leave?

My fiance and I get along well most of the time,we know each other quite well. We had some major problems 2 years ago, where I nearly kicked him out as he was being verbally abisive and intimitating. We have worked through that and he is mostly great to me. However, every now and then he over-reacts to the smallest things, which cause me to doubt the realtionship. We were at a friends place over the weekend and I was talking to them in front of him, there was no way we were leaving him out of the conversation, but he cracked it because he felt he was being excluded and walked of (bearing in mind, we are in THEIR house), saying that if he was being excluded, he would just go read a book. I had to go in and reassure him that he wasn't being left out. He became abusive and irrational, saying that when I left the room to go to the toilet, our friend had walked into the next room and closed the door behind her, thus excluding him again. I did explain to him that it was her house and he was over -reacting, but he just went on and on and on!!. I found myself agrreing with him just to get him to shut up. I am now remebering how I have had to be strong one all throughout the relationship, every time there are dramas or upsets, I am the strong one. He also seems at times to have to be the centre of attention, complaining that often he feels like a "stale bottkle of piss". If peopel talk and look at him ALL THE TIME he is fine, but if their attention is on some one else (especially me), he gets upset. I do try to talk to him, but he seems to think that every one but him is to blame for his actions. I would really appreciate some input or advise. I am at the end of my tether with all these over-reactions!