She cheated after a 4.5 year relationship?

My gf of 4.5 years cheated on me and told me she was drunk. I held her in my arms and told her it was ok and that I forgive her. She said she can't be with me Coz she feels guilty looking at me. I told her I want you to forgive yourself because I have and I'll always be here to help you get thru the guilt and I'll never talk about this or throw it in your face when we fight. She still didn't come back... I kept trying and a few weeks later I find out that she wasn't just cheating on me. She was in a relationship with that guy and never even cared to break up with me. I still wanted her back because I wasn't ready to let go of 4.5 year relation so I spoke to her. She said she still loves me blah blah and is confused about the other guy. I asked her to take a month off and don't talk to any of us and decide for yourself. This way even if you don't come back to me I'd feel that atleast you didn't throw away our relationship like it didn't mean anything to you. She couldn't even do that for me. I get it that she doesn't have any feelings for me but how could someone after such a long relationship not even feel guilty of doing this to another person? I'm trying very hard to move on but I am so confused as to why am I unable to move on. Is it because I still think I love her and can't see the horrible things she is doing to me or is it because I'm upset that she doesn't even feel a tad bit guilty for doing this to me. I'd understand if we were just dating. But we talked about getting married and having kids all the time. She was the kind of person who could never cheat atleast thats how she presented herself for the four years. Sorry for ranting so much. Can't share this with my friends or family Coz I don't want them to judge her. I'm happy that she found happiness someplace else. Just always hoped that if we broke up it would be a while before she was able to move on.