In a huge mess please help

So I recently found out I’m pregnant, I’m engaged and have been in my current relationship for over a year in a half, he was my first boyfriend and everything’s been great. Now I know it’s probably not the best time in mine or my fiancé’s life for a baby but stuff happens and we’re accepting it. On the down side I’ve just found out today that the father of my unborn baby and the man who put a ring on my finger cheated on me multiple times before and after we found out we were pregnant. I deserve so much better so my first thought is to leave but I want whats best for my child, please help I’m not sure what to do in this situation!!