I don't wanna go into it, but there is such a person I feel this way about in my life. As far as screwing it up, it was entirely my fault, and it's hard to get over, as it wasn't rejection, it was me just not pursuing due to my own lack of confidence and circumstances. Now she's married. How I feel about is much different of a situation than, if, say, a girl rejected me or whatever (which has happened before) as it's much easier to rationalize it and go "well she was a b***h anyway" or something to that effect. I can't do that with her. I did end up telling her how I felt somewhat, when she invited me to her wedding and the jig was up, so to speak. I didn't end up going.
It's been almost a year, and I feel still ruined by the whole thing, like the entire course of my life was altered by what I did. But life does go on, I guess. I think the madness that comes from "the one that got away" is what propels all the world's greatest inventions and accomplishments. Without that, those people would just remain happy and not care about building death rays all day (Nikola Tesla.) Maybe in all these great people, this loneliness propels them to do what they do.