How to Deal with Guilt and Shame

I am writing because I have been feeling like a horrible person. I am 25 years old and I have been with my partner for 4 years now and I feel like I dedicated my life for him. When I met him he was going through a hard time mentally and I feel like I stopped living just to see him do well. At first I felt like I loved him and I tried everything that I could to see him happy no matter what happened to me. Well turns out that I have been giving so much and never got anything in return. He started going out and leaving me alone, his family started to get involved in our relationship and it got the best of me. I have talked to him about us breaking up and going our own way but he says if I leave that he will be the same as he was when I first met him. So I haven’t left because it hurts me to see him hurt. but deep down I know this relationship is not for me.

Well lately I have been seeing another guy behind his back and I feel so guilty and ashamed of it all. I have tried to stop but something doesn’t let me and I have no control I know it sounds stupid but I feel like I have no other option. Where do I go from here? I feel trapped, I feel ashamed of myself, I feel like I am a horrible person but I don’t know how to change, what to do. I have been through so much in this relationship with my partner I have lost complete respect for him because of all the things he has done to me. But I also feel like I am the bad one now because of what I am doing. I feel desperate. Please help me.