How long does it take to get over an ex?

I don’t consider myself the sentimental type. I’ve dated plenty of people, but only had one relationship where I had really strong feelings for the person. This was five years ago when I was 20, and part of the reason I ended it was because I could tell I cared about him much more than he cared about me, and it was just a painful situation to be in. It took me a long time to get over him, but I eventually did. Well, I ran into him today and everything came rushing back. He still looks great and he has this girlfriend with really big teeth. Is it normal to still harbor feelings for someone a long time later, even though it didn’t work out? What can I do to feel better? This is really frustrating.