It's my fault

My girlfriend always helped me and was generous but she had her exes name on her body and she always talked about how girls laid in her bed .I'm so upset because she always put me on hold and always claimed that she had other things to do at fix her car which I understand but she kept putting me on hold every day. Today I felt really upset about what she did she bought me stuff and everything but we never had a full conversation within one day she kept hanging up on me and call 8 hours later she's did everything for me she's really nice but I was so upset about how she sometimes mistreats me and she always said that she would change but she never did so today I went to my ex's house and I felt really angry so we had sex and I felt so bad that I had to tell her the truth that I cheated on her because she always kind of mistreated me even though she was really kind towards me buying stuff for her and she bought stuff for me. So it's my fault everything is my fault.