How to move on when children are involved

So I’ve been with by boyfriend for almost 5 years. We have one kid together and another on the way. He use to talk to this woman on Facebook and they said that they were friends. I had no issues with it until I noticed that they were talking at weird times of the night and I expressed to him numerous times that I didn’t want them talking anymore. So I reached out to the girl and also expressed the same and she ended up blocking me on her page and they I see that he was still talking to her and always mad excuses when I brought it up. So fast forward, I saw a video of him in bed with her at a hotel room and he claimed it was a work trip that was in the same area she lived and she just happened to stop by the hotel. Clearly that was a lie. We got pass that and I let it go ( stupid me, for the sake of our son). So recently I looked through this Venmo account and I see that she has been sending him money. And the most recent when I had asked where he got this money from he said his mother sent it and I knew it was a lie I just didn’t have proof at the time. I don’t know what to do anymore. My relationship with is very emotionally draining, I want to leave but I feel sooo stuck....