Cheating boyfriend....

So I am 36 days away from my due date and on September 12th I found out that my significant other was still sleeping with his ex wife. He and I have had our problems for a long long time and I in all fairness contributed to some. I desperately wanted to fix it with him but left him for the sake of integrity. He begged me to come back he got down on his knees and said how differently would be. I finally gave in weeks later and here we are right back at it again he has no concept of consequences infact he believes that when he admits to being wrong that the world starts fresh and he just gets a new chance to screw everything up and of course this comes just a few days ago and it is all my fault. I can’t take this yet I do. Please tell me how to step up and leave this shit behind.