As someone who was in a similar place a short while ago, I want you to know that it's okay to feel how you feel, and don't let anyone or anything try to convince you otherwise. It's important that you communicate these feelings to your gf. In my experience I really had to flesh it out to my partner, because he couldn't for the life of him understand why I was having such an intense adverse emotional response, and neither could I, but I needed an understanding between us so I could move forward.
Sometimes people aren't able to see past themselves; it happens to all of us. A big difference with you is that you're also dealing with trust issues due to this stemming from an affair, so a lot of patience, understanding and acknowledgement on her end will be necessary for this to work in any capacity, and if you need to say it out loud as such, do so.
Examine, layer by layer, for as long as you need to. You'll learn a lot about yourself; I sure did.