Bf/Ex-bf has done something I can't seem to let go of... What do I do?

So wait... you broke up with this guy, and then later, he had sexual contact with someone else? That's not a betrayal. When you break up with someone, their commitment to be exclusive with you has ended. You have no basis to trust him not to have sex with other people - he has no reason not to.
His seeing other people is not about you. At the time, I'm sure he just wanted to get on with living his life.
You got Hinge later, went on a date, and kissed a guy - excellent! Of course you did, you're completely within your rights to - you're a free agent.
"Now he is trying to get back together" - I agree you're taking the right option by not getting back together with him... what you had with him is broken now, and trying to make it work again will just make things even more blurry and poorly defined.
Neither of you are really in the wrong. To be fair I can see how this is really painful for you. The end of a relationship, and seeing your ex begin to see other people, can be awful. It's just a period of very difficult emotional work that you will need to get through. It does get better.
If I were in your shoes I would seriously consider cutting off all contact with him - any continuing contact will very likely just be painful and triggering for you, and frankly, his future romantic endeavours are none of your business. This is your opportunity to get in your own lane, where you can eventually feel happy!

I understand that it's not directly a betrayal and I feel like it's less what he did but WHO he did it with. And how rude she was when I asked her about it. I think this is what is troubling me so much. He does say that it was to get him to move on because I had left such a void in him but I feel like we were eventually going to get back together, from the on and off texting over summer. I feel like he has completely changed as a person too and it's upsetting because I wish he was still the man I fell in love with. I don't know how to let go of this betrayed feeling and I feel like I have extreme trust issues now because of this. Unfortunately, we are in the same community so any type of event I will see him there so although I can block him on everything, I can't block him in real life.

I understand that it's not directly a betrayal and I feel like it's less what he did but WHO he did it with. And how rude she was when I asked her about it. I think this is what is troubling me so much. He does say that it was to get him to move on because I had left such a void in him but I feel like we were eventually going to get back together, from the on and off texting over summer. I feel like he has completely changed as a person too and it's upsetting because I wish he was still the man I fell in love with. I don't know how to let go of this betrayed feeling and I feel like I have extreme trust issues now because of this. Unfortunately, we are in the same community so any type of event I will see him there so although I can block him on everything, I can't block him in real life.

Re: rudeness - Put yourself in her shoes. If you had sex with a guy and then his ex-girlfriend asked you about it, that'd be pretty weird and off-putting, right? I know I wouldn't be predisposed to politeness if it were me... Again, it stopped being your business when you broke up with him.
"Trust issues" - as far as I can tell he didn't cheat on you or do anything untrustworthy while you were together, which is the most anyone can ask. If you feel you would find it difficult to trust your future partner, that is something you'll need to work on - you're far from the only one in that position and it is a really difficult thing. I do feel for you. Ultimately it comes down to accepting that a relationship you get into may end, and after it ends, they will pursue other people - internalising those facts, even before you take up with someone.
"In the same community" - that makes it very hard. I'm sorry to hear it. Do you mean that your family knows his family and you go to church (or equivalent) together? Or that you're in the same friend group? This might be a good opportunity to reach out and make new friends, and spend time with new people. Having some new experiences and new faces around you will help you move past this time of unpleasant feelings in any case.

@anosmianAcrimony has pretty much summed it up.
The only thing I'll add is that if you take off the rose-tinted glasses, the 'man you loved' is someone who didn't prioritise you, nor could you wait to see if he could change; that's why you finished with him. Your connection wasn't even strong enough for him to want to do things differently whilst you were together. Why would you want to rekindle that and how do you know he's changed from only speaking with him; that's just words? What could be the next big problem between you and how would you both handle it?

Thanks for adding to this. I did bring up concerns towards the end of the relationship but it unfortunately did not change. You are right though, he has only ever been all talk, his actions did not show it when it mattered the most. I feel like it is better to leave things but I'm having a really hard time letting go. He wants to be friends at the very least - do you think this is a good idea or forgetting his existence would be a better way to deal with this?

I would be civil to him if I saw him, but wouldn't be a friend; nothing to do with the sex, but if he couldn't prioritise you in a relationship why would he do it in a situation where there's less commitment? I think he'd still be flaky on communication and meeting up.

I'm too petty to be civil unfortunately 🤣 but absolutely EYE opening, you're so right thank you.