Not an ex, but there is a lost opportunity I have beaten myself up about a lot. I was 13 (yeah, probably wouldn't have lasted anyways), everyone was awkward at that age, one of the prettiest girls in class obviously had a crush on me but I was too dense to realize at the time. During recess I went with my friends to all sorts of weird places where nearly nobody else went, suddenly she just showed up where I was, all alone and out of nowhere. She always kept closest to me and on a few occasions she was even putting her arms around me. I didn't do anything about it at the time, it was nice and all, but I didn't understand for some reason.
It took a few years before I suddenly remembered that happening and at that stage knew 100% what actually happened. Since then I have wondered what could have been, even if just temporary. I couldn't fathom how I could be THAT fvcking dense, it's not like the concept of attraction was alien to me at age 13. Worst thing was that I really liked her too, I was just retarded.
Silly story, but the fact that I still remember this so vividly at age 29 says something about the impact it had on me. I wish I could explore that alternate path, how different would my life be if I didn't blow the chance to be with the hottest girl in class? It would be interesting to know that, but I've let it go a long time ago. I still cringe thinking about my complete inaction towards a hot chick I like who literally throws herself at me...