I'm Dr. Jackie, an interpersonal communication researcher/professor and relationship expert. And let me just say--please do not think you are alone or that you cannot be helped. Everyone, and I mean everyone including THERAPISTS and relationship counselors, need to talk to someone at some times. No one is perfect--we all need help with something. So please do get that thought out of your head. you are not "helpless" in that everyone can be helped.
Second, I don't want to substitute for face-to-face counseling. But we do offer phone and Skype support that may make it easier versus this Q&A/email type format. I'll send you my information via the JA form after I am finished here.
Third, you are not reacting in any way different or abnormal from what I would expect someone growing up with the situation with your parents. The fact that you still have a girlfriend and appreciate her and are so sensitive to her feelings reveal your capacity to care, love, and feel emotion--these are VERY GOOD THINGS. So if ANYONE can learn and grow and be a better relationship partner, from what you have shared, it is YOU!
Now you CAN be faithful to your girlfriend unless the women with whom you have cheated have all held guns to your head. I don't think this is the case. So it's not that you CAN'T, it is more that you have witnessed infidelity growing up, and somehow that is either the norm or else it is a safety net for you for not getting as close to your gf as you could be. You need to think about which it is.
I would suggest working with someone to start rebuilding your securities and self-esteem when it comes to relationships. You hint that you are not being fair to her, and you really are not being fair by cheating on her. If you are not going to stop this behavior, then you definitely need to come clean and let her know. Otherwise, you truly are misleading her. Based on everything else you have shared, I really do not think that that is who you are. The fact that you have shared all of this definitely shows you are sensitive to her feelings.
Please let me know what you want to do--do you want to talk via phone/Skype? It might be easier to talk/chat in real time. If you would like to keep emailing through this Q&A, that is fine, too. It's up to you. I would like to see if we could work on you gaining more confidence in yourself as a good relationship partner. That is where it starts. In other words, through therapy, I would like you to work on being a healthier YOU first so that then you can be healthy for a relationship partner. Does that make sense?
Please let me know what you want to do.
Take good care,