Why does being cheated on still hurt?

TLDR: Why does it still hurt so badly that I was used and disposed of?

I 26f (24 at the time of this happening) was with a guy 28 for about a year. I found out in the middle of our relationship that he was cheating on me. Long story short, my dumbass took him back. Multiple times because I have to make damn sure I’m being as stupid as possible apparently. (And I mean, let’s be honest, growing up in a house with no love and a lot of manipulation can have you mistaking manipulation for love.)
I really thought I was going to marry this idiot, he lived with me, I bought a ring, I was going to propose, etc. I found out he was once again cheating on me, after he took me to meet his parents, my daughter met his parents, all the stuff you’d do with someone you really did intend on staying with (I thought). He ended up cheating on me and ghosting me, after previously asking that I break up with him in person (then didn’t give me the chance to). We’ve obviously been broken up awhile, I’m in a good, healthy relationship, going on a year and a half now.
I just feel sometimes like I never got closure, didn’t get to say goodbye, and despite taking quite a bit of time to heal, it still bugs me that he treated me like such a disposable option. I guess I’m more mad at myself for accepting such garbage and labeling it love, because deep down I knew better. It still bothers me so much that he did what he did, cheating, lying, running around being a little nasty, all while being supported by me and being a part of my family.
How does one get over things like that? I’ve gone to therapy (still am) and really I don’t even know how to voice my feelings about it in a way that doesn’t come off like I want him back. I really don’t, I have a visceral reaction when I think about him.
I guess maybe I just needed to vent, but I also do want to know what anyone who has been through an awful break up has done to cope. I’m out of ideas. And while I would never allow myself to be treated like an option again, it’s always in the back of my mind that I did it once in the first place. I love my current bf, hell, I’m pregnant with his child, we plan on getting married sometime down the road. I just feel like something inside of me broke with my previous relationship, and I want to fix it, because I want to let it go and move on from the hurt.
I’ve moved on from the guy, if that makes sense, but what he did seriously fucked me up. Gaslighting, lies, manipulation, possibly exposing me to STDS because the girl he cheated with is disgusting (I got tested and I’m fine, but I should never have had to worry about it under those kinds of circumstances imo). I just feel like even now, despite everything my current bf and I have worked through, that maybe things will spiral again. Pregnancy isn’t helping with how I feel about things (way overemotional to begin with, so much worse now that hormones are piling up).
Why do people cheat? More importantly, why do people become serial cheaters and hurt others like this? How do I let it go and stop giving it life in my head? I fall down a hole when I start to think about it, and nothing my therapist has said has helped me figure it out/let it go. I’m hoping venting helps, or finding some solace in others who have experienced the same thing. If you’re still here reading, thank you for taking the time. This was a giant rambling, venting, post. But I am genuinely open to advice on how to cope♡