LADIES HELP, NEED RELATIONSHIP ADVICE

Usually I would go to a friend on this topic but I figure maybe there was someone who was in the same situation as I am in. My baby's father/fiancé has been cheating on me since December & I don't know what to do about it. We're in the awkward stage between figuring out wether or not to stay together. Last night I found messages between him & at least 20 other girls & he told me he actually had sex with one yesterday morning when he was supposedly at work. He says that it comes from a place where he felt insecure about our relationship & he admitted to looking for constant attention from other girls because our relationship has changed after baby. Obviously relationships aren't the same after a baby, my son is currently 8 months old & I have no idea whether or not I want to fight for this relationship anymore. Anytime I look at my fiancé I get disgusted & I get reminded of what he did. I keep looking at his Facebook messages & getting angry over & over again. 

Even if I do continue with this relationship there is not going to be trust like there once was, I know that I'm going to be constantly thinking about his cheating past for who knows how long. Has anyone had a successful marriage after your significant other cheated? What should I do? What have you done? What would you do? Please give me raw, honest answers, disregard my feelings. 

Thank you in advance!!❤