Honestly, if I were you, knee jerk advice is to break it off.

Like I said, I've been in the same situation before, more than once... best case scenario is essentially:

You somehow get back together, but you will not be able to fully trust them and will always feel inadequate.

Not a good set up.

Doesn't have to be messy, dramatic, ... hell, you could just ghost him back.

But yeah, I'd say the sooner you try to accept that this isn't likely to work out, you're hurt, he doesn't seem to care... sooner you get to processing that, the better.

However, I don't actually know all the details, there potentially could be some important missing context.

I am assuming that you two have been like, a monogamous couple for something like 3-6 months + ... maybe no, maybe its a significantly different situation?

Maybe think it over for a day or two?

Try to get to the point where its not the predominant thing in your mind, or to the point its not in the back of your mind?

Always helpful to try to have a clear head, step back from the situation as much as you can.