Being with someone who got cheated on

So. My ex got cheated on by their previous partner. When we were together things were so great, and we were getting to that comfortable place in the relationship, but then out of the blue they just break up. No real reason, just a bunch of weak excuses.

I think my ex was scared that I might cheat or hurt them in the same way, and that's why they left, but I don't know for sure.

I want to reach out and talk about this more. I think they would be receptive to it, but I also want to give them as much time as they need to figure things out on their own. I also don't know how to go about reaching out without devaluing my own self worth by running back my ex.

Any advice would be great. Thanks!