Cheated On Several Times. Why Am I Still Here?

Long Post: I've been with my SO for almost 6 years. We have been engaged since the 1st year we met and we have a 16 month old son. A few months before I found out I was pregnant my So became an alcoholic. He would drink until he blacked out and would become like a stranger I had never met before. He was verbally and emotionally abusive and began pressuring me to do sexual things I am uncomfortable with. He would get drunk and say and do terrible things then when he woke up in the morning, he wouldn't remember. I have lived with my grandparents and me and our son only really hang out with him and stay the night on the weekends.

When our Baby was 4 months old my So told me he was going to AA and they told him they didn't think he could do a serious relationship while he was trying to stay sober so we "broke up" but he kept trying to reassure me that he still felt like he always did. it was still like we were together. We still hung out and spent the night and had sex and everything just like when we were dating. When he told me he was talking to someone else and had feelings for her so much that he felt like he was cheating on her with me. I later found out that it was one of our friends who use to come over and watch scary movies and drink wine with us every weekend. He told me he had cheated on me several times with her and that she "seduced" him the first time by getting him to drink with her and she asked if he wanted to watch porn with her and came back in the room in slutty clothes. They screwed 3 or 4 times and she told him it was a mistake and to delete everything they sent each other and now tells his family he raped her.

We are together again and have been for almost another year and a few weeks ago while he was drinking, he confessed to me that he cheated on me before her with 2 other women multiple times while I was pregnant because I am uncomfortable doing anal. He said he paid both women to let him do anal.

I'm having trouble letting myself think about all of it and processing it. I just feel numb. I'm still with him. I Love and care for him and I want us to be able to fix our relationship. I just feel numb and like I'm shutting down when we have an argument and when we're together. His drinking has really messed me up over the last year but I feel like when he isn't drinking and when we can really enjoy ourselves when we hang out he's my best friend and the man that I fell in Love with all those years ago. Other times I feel confused and upset all the time when I'm there.

Last week a small painful wound formed on the inside of my lip and I went to the doctor thinking it was a canker sore but they said it's a mouth ulcer caused by something viral. They didn't do anything but look at it then put me on antibiotics and antiviral meds. In a few days another formed on my upper lip and causes my lips to swell some. Sometimes I think about what if it could be some kind of sexually transmitted thing because I never knew he cheated until he told me. He's still pressuring me to do anal although I'm uncomfortable with it and I know he has cheated on me before and I have no way of knowing if he does unless he tells me or I find out about it.

Hopefully I'm worrying over nothing.

I just don't understand what's going on with me how I feel or why I feel so numb