My Ex-Husband's New Love Life

I recently got divorced after six years of marriage. My ex-husband moved out about two months ago and within a week he was already seeing someone new. I didn't mind too much at first; we were both glad to be single again, even though it stung initially. He's forty now, older than me by a few years. We never had kids so the breakup wasn't as dramatic as it could have been for others, but emotionally we both needed our space and freedom. Fast forward to last weekend when I ran into him at a local bar while out with friends. It turns out his new girlfriend was there too - in her twenties, fit and very put-together compared to the rough-around-the-edges me these days. They seemed happy enough together but it made me feel old suddenly. I don't know if I'm being petty or immature for still feeling hurt about this so long after everything ended between us. We agreed to move on amicably, yet seeing them made me wonder what he sees in her besides the obvious physical difference between us now. Was it wrong of me to have felt a pang of jealousy? Or was I just grasping at straws and hoping for something that could never be again? Am I being too harsh about his new relationship so soon after ours ended? What do you think - am I the asshole here or should he have kept things more discreet with his rebound romance?