I’m the other woman

I caught feelings for one of my friends who has a girlfriend. We would flirt but it never lead to anything and I wasn’t planning on doing anything with him especially because he has a girlfriend and he says he wants to marry her. One night I got very drunk (he wasn’t) and then we had oral sex. I regretted it especially because I thought it would destroy our friendship. I talked to him about it the next day because I was worried and I told him that I understood if we could no longer be friends and I told him to block me on everything because I assumed he was going to tell his girlfriend. He never did. Time went on, and he started to become a different person. He was rude to me and I would find myself in situations where he wanted to do it again. I never said no, because I have a hard time with that and deep down I knew that I wanted to continue to do it. The more we would “hang out” the more he would say mean things to me and treat me like dirt. I haven’t seen him in a month and I’m trying to move on because I knew it was wrong. I am just filled with guilt because he has a girlfriend and he still talks about marrying her. I don’t know what to do. I feel like she has a right know but I also want to keep my friendship with him. What should I do?