Feelings for an ex?

It’s been five years since we dated and we both had our own problems going on during puberty and high school. But now as a jr college, I went to a party with some old friends and he happened to be there. We’re on good terms to speak and hang around if we’re together but he’s dating my high school bully for about a year and half now.. I thought I was over him, or maybe these feelings will pass soon. But how would you cope with experiencing feelings for an old ex again?