It sounds like you are not healed from being cheated on previously and so are now looking for/expecting it to happen.
You don’t mention too much context of what happened with you ex-H but if that was a total shock at the time within the confines of wedlock then it is natural to think that if HE could do it (after marriage/kids) then it would be definitely something someone who was not that “officially” committed would be capable of.
You are trying to spot it before it happens and seeing things that aren’t necessarily there. That is understandable (although I wouldn’t be happy about the arm around the same girl in every photo at the event either tbf).
I’m so so sorry you feel like this - you are definitely not alone in feeling like this. It’s shit but it is a mindset - YOUR mindset - and you can start to be far, far kinder to yourself. It might be hard but you have 4 beautiful DC to whom you are the most important person and it sounds like you’ve been through a very difficult time. I think you should try and slowly grow an inner voice that treats you with the respect you deserve.
Every time you hear the inner voice starting to slag yourself off try and imagine that voice is using those words against your mum/best friend/child. How would you react? Tell them to leave them alone and F off? Say it to the voice. Don’t let it drag you down.
Start a new voice. One that says “I am an amazing strong woman who had the strength and balls to leave my cheating, [insert adjectives] exH even with 4 DC”. THAT is bloody impressive. You’ve surpassed Beyoncé there!!!
You deserve to be loved, respected, valued and treated well. You should start treating yourself like this.
If you’re dwelling on other random girls this undermines every other quality you’re bringing to the party. And no matter how much you don’t want to admit it, you must have something pretty hawt going on as it sounds like this guy really, really likes you.
He is not out of your league. You’re in a relationship. He’s not better than you. Please be kinder to yourself. You sound like a lovely strong woman just in a cul-de-sac of self-loathing and you deserve better than spending your time punishing yourself and putting yourself down.
My mum used to say “the only person you’re guaranteed to wake up with every morning is yourself” - please be a good, loving friend to yourself.
Good luck OP xx
