Have you ever missed an opportunity and regretted it? If so, what happened?

I have, I was at elitch gardens (amusement park) yesterday and I found the guy of my dreams. I've never believed in love at first sight, but yesterday was literally the perfect definition. Oddly enough we were about to leave, but then a firework show started and we were under twinkle lights. I looked over and I was standing next to him and my heart totally fell into my chest. I wanted his number so bad, but I was super shy, although I'm a very bubbly person. It was so strange. But, everything was like perfect. I even bumped into him once and felt sparks. Now I can't stop thinking about him... and I feel so bad. Most regretted thing ever.Have you ever missed an opportunity and regretted it? If so, what happened?
There was this boring class at my University.... but from the day I noticed that girl, I wanted to attend the course and do well. She was from Slovakia and I (am not an American btw) totally fall in love with her from the first moment. Time passed and finally I got her number, knowing very little about her. We were hanging out as time was moving on, in cafes, and bars and everything and we got really close. I could see she liked me and she enjoyed being with me and we were having good time together. We kissed and it was like Heaven... One day all of that changed in a matter of seconds. We were in this bar and we were talking and laughing with her and some friends, when I heard from her mouth that she is awaiting her boyfriend to visit her in a couple of months. At that time I froze, I was really quite for the rest of the night, and I did not know what to do. I clearly stated to her that I don't support this kind of things, and that I wasn't feeling good about it and so I put an end to it. This was our last time we went out together. My life taught me to be a gentleman and decent, and I could not do this to her, because yes we were young and all, but she already had a boyfriend, and she wanted to be with him, I just did not want something wrong to happen to her for me. At that time, I thought I was doing the right thing and I was pretty sure of it. But how come right things hurt so much? 5 years have passed now and not a single day passed by and not felling the bittersweet taste of regret. I never stopped thinking about her...