I had something with a guy who happens to be a good friend of mine too. It started when neither of us were in the right place for anything serious but it soon snowballed. We never became 'official' though we both acknowledged that we were a couple and the few who did know what was going on said we were very much a couple.
Anyway, after an argument he went out for drinks (already arranged) and ended up kissing someone we both know who also likes him. He's always said he has no feelings for her and that night he did it because he was angry. From then on I had a problem trusting him. I was paranoid that he would go off with someone else, and this was not helped by him talking to loads of women. He's always had plenty of platonic relationships with women, but that thought still didn't put me at ease. So... I ended it.
I was distraught at first but we actually managed to slip into our friendship only relationship really well. I was immediately relieved and no longer paranoid. Things were great!
And then... one drunken night we slept together. It's then happened several times since. The first two times I were fine, I told myself it was nothing, but now I can feel the paranoia creeping up again. He doesn't want to commit to a relationship again right now (from what he's said in casual conversation) and I'm at a place where I need someone loyal to me. I know I need to say no to him, we are amazing as friends but anything more is just damaging right now. I just don't know how I can do it? Where do I get the strength from? He's my best friend and I can't lose him as that, but as great as the sex is, I can't do it again, not under the current circumstances.