How should I end it with this guy?

I know this may be a dumb question but I need a good answer. I started talking to this guy and found out a little later that he had just broken up with his ex. He kept telling me he was over her and I didn't want to completely stop talking to him because I had previously liked him and wanted to see if maybe this could go somewhere. We hung out a few times and it was really great. However, I recently found out that he met up with his ex and now I truly feel like an idiot for catching feelings. I know that I should probably get out before I actually get hurt. But I don't know how to actually end it without having him know that I found out he met up with his ex. I want to stay kind of mysterious and as if I don't actually care about him. What can I say to him that would get me out of this situation without being bitter/rude so that there's no animosity and we could just be casual acquaintances? For reference we are both 20 years old.
How should I end it with this guy?
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