I'm going to speak on the other side of this. I believe very much that people can fall in love with more than one person at a time, and not know how to deal with it properly. I was involved in an affair (I was the other woman) and I feel like hell about it now. I lost two very good friends out of it, and I don't think it was worth it at all. That being said, I know the guy still truly loved his gf, and although the affair went on OVER A YEAR, he never stopped loving her, despite also loving me. When push came to shove, he found out he was also a sex addict, and has very bad impulse control.

My advice, talk to your gf. If this is the only thing she's really been lying about, and is willing to get help and work on it (she might also be a sex addict?) then I see no wrong in staying with her. If she lies to you about a lot of things, or hurts you in other ways (emotionally, or manipulating you mainly) then seriously consider leaving. You will find a way to live without her.

Most of all, don't blame yourself. I doubt very much she did this maliciously and unless it's obvious to you that she is using you for money or protection, I don't think she would have asked for forgiveness if she didn't still want to be with you. Whatever her reasons for this though, it is her reasons, not yours, and you should not have to take responsibility for what she did. It hurts I know, and I know you feel betrayed, but it isn't your fault. At all.