I feel Stuck

So me and my partner has been together since the beginning of September in 2019... it was rather quickly after I left my husband off 6 years due to cheating.... I know I shouldn’t have rushed. Me and him actually meet during my ex husband and I first fall out when I left due to cheating. My partner and I decided we wanted to both go back to our exes. Anyways. In the beginning of the relationship he still was hanging out with his ex I blew it off because we just started talking etc. I then found him at her house when he was suppose to be going an hour and half away to look at a tractor. I went to leave and he came home and stopped me.... he told me if I left that was it and his biggest fear was to be walked out on. I felt guilty, he said he was there to get his stuff from her house etc... fast forward to probably April of 2020.... he bought a car the kind he collects from someone who happened to be a female. I shortly then noticed him messaging her often to find messages where he was interested in a relationship with her etc. I again, confronted him and was going to leave to then be told “I was just getting a feel for if she would be a possibility for a threesome” as we have talked about doing that... I gave in, again but told him that he should of let me know prior, I should have gotten info or seen who she was and be in on it to. He quit talking to her.... now fast forward to September 2020, I was in the process of selling my house and he introduced me to a girl who needed a place to stay, she was getting out of an abusive relationship and we felt the need to help her. She was also interested in buying some property we were investing in prior to leaving. Well..... I got in a bind this past week where my babysitter just fell off the face of the earth and I needed a new one. The girl offered. She came and then the night before last realized that my partner and I was not just “friends” and informed me that October and November of 2020 he was trying to be FWB with her!!! I have all the messages and I confronted him and he first denied it, said they never went out to eat, and that she changed the names on her phone it wasn’t him. He then said it was because she was a threesome possibility bullshit again too. I told him I didn’t believe it because he was offering her his bed to sleep in the apologized saying he shouldn’t have said that, then he even said how he wished they would of gotten freaky together because God knew he needed it etc. I went to go leave and he told me if I left, that it was done and I got scared... he came home from work and f’d me and is acting like everything is ok and I’m hurting because I know it’s all a lie but the thought of leaving and restarting AGAIN with my kids scared the hell out of me. Now I’m lost on whether to just still run or figure it all out. 😭😭