Ex-Wife's New Partner?

I got divorced a few months back and I'm mostly okay with it. We had our reasons for splitting up but we still remain cordial, at least on the surface. One day last week, I ran into her at the supermarket. She looked fantastic as always. We caught up briefly, and then she mentioned that someone new was in her life. This new partner is apparently a lot younger than us and my ex-wife seems genuinely happy with him. I know I shouldn't be feeling this way, but ever since finding out, I've been struggling with feelings of insecurity and resentment. Part of me wants to reach out to them both for some closure, but mostly it's just this gnawing jealousy. My ex-wife mentioned that he was good at helping her feel comfortable and happy again after the divorce. I'm not sure if it's normal to feel like this. Maybe I'm just being immature or holding on too tightly to what we had before. Should I try talking to them? Or is it best to leave things as they are? Am I the asshole for feeling this way?