I think maybe your girlfriend is confused.She moved on to another guy to maybe try and forget how she feels about you, but still wants you in her life because she still cares for you. When you went out with another girl she got upset maybe because she thought you would be more 'loyal' even after she hadn't exactly been the same to you. Girls can set unfair expectations, and set you up to fail without realizing it. I have done that before, in a way. Breaking up with a guy and saying I want you to date other people, but all the while had a HUGE knot in my stomach saying it, but didn't expect anything to happen, and when it did it is like a shocker in a way. I'm not saying it's right to do this, because it is mean to do it and it hurts you and whoever you are doing it to.
In a way, I think that maybe it is too hard for you to be just friends with her. I know what it is like to try and stay friends with a very close ex. You are always grasping at straws praying that maybe if you are nice and friendly you will remind her why she should want to be with you. It's hard, because usually that person gets comfortable as 'just friends' and you just have to accept things. I think maybe you should take time to get over your feelings for her if you want to be friends...it's just not fair to you if you hang in there and play her games. Maybe during the time you take away from her, she may realize what she lost, and may try to come back in your life. Maybe not. But you need time to move on and not torture yourself.
