[FULL] Husband moved mistress in after I gave birth

My horrible husband moved his mistress in with us a month after I gave birth, because I'm not enough. Now he's begging on his knees and refusing to believe I'm leaving him. Just three weeks after I gave birth, my husband came to me and confessed that he cheated on me. It felt like the ground opened from under me. He apologized and tried to blame it on the pandemic, on being bored at home, on being scared to be a father, on a midlife crisis. But he said that he loved me and he didn't want to lose me. I felt so trapped with a small baby that needed me. No job and no place to go. Good job for him to know when to deliver this blow. A week later, he asked me what I thought about opening the marriage. He said humans aren't meant to stay monogamous, and while he still loved me more than anything in the world, he didn't find me enough. I started crying and begged him not to humiliate me. If he just gave me some time, we could divorce and go our separate ways. He said he wasn't interested in divorce, he didn't want to replace me, just add some new excitement into our relationship. Two weeks later, he came home with her and told me that she's moving in with us. A 26 year old that he met at work. He started showing her the apartment and explained how important it was for me to have everything clean and tidy all the time, and that she should abide by that. She was smiling the whole time. He took her to the guest bedroom and she's been living with us ever since. I moved to my son's room, put locks on and I've been living with my baby for almost a year now, rarely existing in the common areas besides preparing for my catering gigs. The girlfriend moved eventually into my bedroom with my husband. He warned me that this would happen if I refused to move back. After a few months, my husband gave up trying to talk to me outside of matters concerning our son. He finally realized neither fighting, tantrums nor love bombing worked on me, and most importantly, he stopped trying to get intimate with me besides the occasional I love you or I miss you text. Sometimes while he's literally on the other side of the wall. His girlfriend kept her distance from me, and I was thankful for that. She moved out a few times after they would fight. I don't know the details, but I would guess that she wanted exclusivity. Oh, the irony. She always showed up again after a couple of days. I acted like I didn't notice any of it. I have one year left of my studies, but after my internship practicing semester at a big firm, my boss offered me a paid internship. It's a full-time job starting in January. It would mean work and study full time, but I know that I will manage. I'm just so happy. This means that I now have the means to find my own place, and I did. I'm moving on November 1st. I will have to break the news eventually to my husband that I'm moving out and filing for divorce. I don't want to tell him right now, but once I start packing, he will probably understand anyway. I don't know what his reaction would be. Ideally, he would be relieved, but somehow I doubt it. Right now, he thinks I'm bound to him for at least another year because of my studies, and he's still acting like I'm his wife. I have been so lucky this far, and hopefully my moving out and divorce won't be any different. I haven't been this happy in many months, and I wanted to share it somewhere since nobody in my surroundings knows what I have gone through. I got the keys to my little apartment two weeks ago because the previous tenant moved out earlier. I moved last Friday. I had packed all my kitchen gear, my sons and my clothes, toys, etc. While my husband was at work, my best friend and her husband helped me. When my husband came home, I told him that I was leaving. He was confused at first, but I told him about my new apartment. So he asked me how I was going to afford it, his facial expression changed, and he looked scared. He started telling me how he has been regretting all of what he did. He loved and missed me. He knew that he messed up and had been thinking that this wasn't what he wanted. He wanted his family back, our son, and me. He asked me if I wanted my baby not to have his parents together. Then he said that he was leaving his girlfriend and she would be moving out, and it could be the two of us again. His girlfriend started fighting with him and I just left. He has now been calling me, asking to see us. I still want a decent co-parenting relationship with him, so he took our son on Sunday to his parents to spend time with him. My in-laws apologized for what my husband did, and the weirdest thing is that I believe that he still loves me too, in his own weird way. I don't think he ever stopped loving me. I have watched him with his girlfriend for a year now. It's never like how he was with me when we were happy. The only times I could see his old loving self was when he looked at our son or me when he thought I couldn't see him. When I think of this, I cry even more because what a waste of a beautiful relationship and for what.