I don't feel so good about admitting it, because I had a recent tough break-up for which I blamed her, but yeah, I cheated on my previous gf.
I made out with a girl at a The Strokes concert no less. I have to admit it, it was great. But I was in a 4+ years relationship at that point.
The worst part is that I kinda fell for that other girl, while keeping the relationship I was in. Of course, the relationship started degrading, even without she ever finding out. Because I stopped caring, stopped making plans. But the other girl didn't care much for me though, or at least she was very confused, as she sent very mixed signals.
Anyway, while this was going on I was in a destructive phase, and one night going out with a friend, I made out with yet another girl, for who I didn't care though.
Of course, all of this was eating me inside, and sometimes when I got drunk or something I would get emotional and tell my gf stupid things like "omg I have a terrible secret

" or cry about if we ever split up. I cared for her, of course. But obviously not enough, at least at some point.
So anyway, after that episode was almost through and I had cleared the other girl out of my mind, and was sure I wanted to keep my relationship, I tried to come clean. Except I didn't. Well, I did for the most part, which was revealing that I had been having feelings for another girl... because I think that was really the unfair part. Lust is lust, love is love. But I didn't reveal that I had actually made out with her or anyone else.
She didn't receive the news well, as you'd expect. But more in a "well I don't know if I should stay would you, just because I'm not sure if you still love me or if eventually I'll have to compete with that girl". She was just afraid of losing me. But I let her rest assured that I loved her and no one else. And we went through.
But some months later, we had a rough phase and broke up. I tried to get back to her but she wouldn't take me anymore. And I'm sure that episode had a great weight into that decision. The truth is a bit more complicated than that, as she started talking with another guy, who she now dates, before we broke up. But still what happened before contributed to that as well. Because I understand that if you feel like a second choice, and not loved as much as you love someone, you probably will try to find love somewhere else, consciously or unconsciously.
The moral of the story is, don't cheat. If you ever do, man up right away, tell and break up... or if you really really really still love her (and you have to think about it quite deeply) and it was an honest mistake, forget about it, don't ever EVER tell her or anyone and don't do that shit ever again.