I hope I'm not going to come off rude here, but the answer to this is pretty simple. If you can't get over something she did in the past that had nothing to do with who she is now - then you need to end it. All you're doing now is stringing her along for your own gain when your heart's not in it. You already said you won't put a ring on a finger of a girl who's had a three-some - so why are you still with her? Are you afraid of being alone? I think you need to end it, not only for your sake - but for hers as well. Good luck!