Why do I cheat on my wonderful bf?

Ever since my first "serious" relationship (4 years ago), I have cheated on every single bf I have had. As bad as it sounds, it really hasn't bothered me too much until now. I am currently dating, and living with, the nicest, most affectionate, funny, GOOD guy I've ever met in my entire life and I can only see the future being wonderful with us...unless, I **** it up. For a few months now I have been having an affair with a friend of mine. Although we've only actually had sex once, almost every time we hang out, we kiss. I didn't plan for it to happen, I just couldn't control myself. Cheating is like a game for me, and I almost feel like I'm living two separate lives. The friend I have been having an affair with and I both love going to electronic music shows, that in which my bf doesn't enjoy very much. I don't know why I do this, but if I don't, I'll lose the love of my life and never forgive myself. I've tried to stop talking to the other guy, but I feel like I'll lose a good friend if I completely shun him. How do I stop this? and why do I do it in the first place?