I found out my boyfriend cheated in the beginning of our relationship: Advice?

I just got engaged to my dream guy. He loves me and my two kids in ways I didn’t even know existed. I grew up in a toxic poverty-stricken family and ended up having a string of long-term abusive relationships. So, when I met my fiance, who comes from a good family, I feel like he taught me how to love and what love could be. Recently though, I found messages from a relationship of his that overlapped the beginning of ours. All in all, before we moved in together but were dating seriously, he cheated on me several times. I even gave her son a ride to a friend’s house, not knowing the capacity of their relationship. After we moved in, their contact stopped completely, and he has been faithful to me since then almost two years ago. I’m heartbroken. Just straight devastated. I feel like my happy ending is gone because I can’t trust him anymore. I don’t know what to do. Do I make him leave? Do we work at it? I love him so much but my faith and trust and hope is just… Broken. I also want to confront her since she’s married and was 2 years ago when it happened. But would that just be bad karma? It wouldn’t make me feel any better. I’m just so lost…
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Once a cheater always a cheater