Well, you said that she had sex with this guy three years ago, before the two of you were dating. It's very likely that she still had feelings for this guy, even if it was just the want for sex. That feeling really never dies to be honest. I still get it sometimes, whether I want it or not. It just happens. I do admit that it's a little suspicious that she took time off from your relationship and then had sex. I'm not sure what that's about. Her excuse seems a little suspicious, too. What I think you should do is calmly ask her if she loves you, if she wants to be with you. Ask her about her feelings/opinions of the guy she had sex with. Ask her if she thinks she still feels that way for that guy. If she shows any doubt, then I think it's best that you move on. If she seems really sorry for what she did, then I think you should give it a second chance. You really have to think about what's best for you here. I can tell that you reeeally love this girl, and I'm sorry she hurt you in this way. Sometimes, it's just that wake up call that tells you that this isn't the one. Sometimes it's a test to see if the two of you can survive those mistakes. What determines that is how you choose to handle the situation. I can sit here and keep telling you all this advice, but really it's your decision completely. Good luck and again, I'm sorry!