Most Helpful Guy
It sounds like you want to break up with her. If you are really gullible I think it would be a mistake to break up with her directly. It would give her a chance to use your emotions to her advantage and give her another chance. People say you should break up with someone to their face, but I think that is more when you are breaking up with someone that hasn't wronged you. She deserves no explanation nor any closer. Just text her that you are breaking up with her because you think you can do better, and don't answer if she replies. If she keeps calling block her number.
If you want to tell her why you are breaking up with her, you can, but it will hurt her worse if you dump her and she doesn't know why. She will over think it and wonder if she wasn't hot enough, or if she did something annoying. Ultimately she will likely think it was because you didn't think she was good enough for you.
I wouldn't even let her know she hurt you. She might even do the same and calmly accept that you don't want to be with her, even if she is devastated by it. The more pain either of you show, validates the worth of the other person making the break up easier for them. You don't want it it be easy for her. Then try to get your photo taken with some really hot girls and post those on Facebook in case she stalks you online to really drive the knife into her chest so she feels like she was easy to replace. Don't do it like you are bragging or she will see through it. Make it look casual so it looks like you have just easily gotten over her.
If you are wanting to save your relationship then just talk to her, and take time rebuilding the trust. You however aren't married so I would advise just going your separate ways.