diezach , 22

I've been with my boyfriend for two years now, we live together with a couple friends as well and I love him a lot. But we also fight a lot over stupid things and over things he doesn't like about me like smoking or my friends. We are such different people which can be a good thing but it also means we butt heads a lot. And not only are we really different but we are heading down different paths in life. We've broken up twice over the last two years and we definitely have our fair share of problems and hurtful memories. Sometimes I think about breaking up again. But when we aren't fighting we usual get along pretty well and have fun together.
My big problem is one of my friends though. We met six months ago at my new job and quickly became very good friends.  We get along super well and have so much in common. We can spend hours with each other and it's always great. He is one of the coolest friends I've ever had. A couple weeks ago he told me he really liked me and held my hand and I told him the same. But he has a girlfriend and obviously I'm not single either. I guess knowing this I should have stopped hanging out with him but I can't do it. So up until a few days ago we had been good when hanging out, just friends. But the other day he kissed me and we tried to stop but once it had happened once it was so hard to resist how good it felt and after an hour we ended up kissing more and then later having sex. I feel so guilty and I did at the time as well and so did he but it was so amazing and felt so right that I couldn't stop myself. And now I can't get him out of my mind, I already thought about him enough before this and now he's making me crazy.
Now I don't know what to do. On one hand I love my boyfriend despite our problems and I want to be with him but that means that I would have to stop hanging out with this friend and we have such an amazing connection that I don't know if that's something I could do. We almost broke up last night over a stupid drunken fight and while I was really sad today before we worked it out I couldn't help thinking of the prospect of being single. And when my friend told me that he almost broke up with his girlfriend a couple nights ago I couldn't help thinking about being with him. I'm happy me and my boy worked things out but at the same time I don't know if it's for the best. I feel like such a horrible person right now. I don't know what to do with myself, any advice would be greatly appreciated thanks.