What would it be? Would it be a crush you felt you should have asked out? A relationship you feel you could have saved? Its really not to good to be looking back on the past like this, but sometimes it can be fun to fantasies about what could have been.
If I had some way of going back and time to change things, I feel I know exactly what it could have been.
There was this girl I have known for the past four years or so, she was the first girl I had ever told I liked. Thats literally what I did too, I didn't ask her out or hit on her, I bluntly said "I like you". It was the boldest thing I had ever done and I completely regret it. These types of things are supposed to have happy ending when you try and bust out of your shell, but she absolutely refused me.
If I could, I would go back and never say that I liked her. If she ever asked I would say "as a friend". I have remained good friends with her until very recently when I confessed a lot more to her, but she has now taken it the wrong way and thinks I am trying to steal her from her BF. Because of making that confession all those years ago, it has caused a lot of pain. My romantic life during high school (which is now ending) never worked out at all and I now just want to forget she exists and restart my social life in college in a few months.
Ok, mention yours. You can have a long drawn out description of why (like me) which would be sort of preferable or you can write one sentence.