Help me get past fiancé old hookup. I’m upset

First off, I posted this a couple days ago and didn’t get as much help as I was hoping for. I will try again

Okay. it happened before we knew each other... I guess they had hooked up a few times and then she ended up moving away a couple years before he met me, but I just learned that he had been talking to her again on Facebook briefly when we were just starting off together. It was only Facebook, but I am still so upset. Hear me out on this...

before my fiancé and I started dating officially, like “talking,” I noticed this gorgeous girl (literally perfect body) all over his Facebook, commenting and liking everything. At the time I asked him who it was and he said just a coworker and then immediately deleted his Facebook and that was the last he has talked to her.

Long story short, I found out about this girl and her name, and found out that she wasn’t his coworker, she was an old hookup. I know he never “cheated” on me because she was a couple years before he met me, but ugh it bugs me he was talking to her on Facebook when him and I were just starting off dating.

Basically what I’m saying is, I can’t get this girl out of my head, him and her having a thing.. I’m so jealous. She is SO freaking pretty.. good lord.. it keeps popping up in my head driving me crazy. I’m so jealous. I want to not think about it. Just the thought of them together kills me! 😩.

If they hooked up in the past, I can let that go, but the fact they were talking for a bit on Facebook when we first started off together.. and he lied about who she was.. bugs me so bad. Am I overreacting? Why do I feel so upset?

Please help. I have no one to talk to. Would you be upset in my shoes, like if you found out he was talking to this girl in the very start of your relationship?

Also, I apologize if this post is all over the place. I’m sure y’all get what I’m saying. I’m hurting 😞