Do I have to pick just one?
It might have been professing love dramatically, like in the storybooks, and then getting frustrated at her noncommital response(she actually didn't reject me, in retrospect), and having to spend the rest of the class in uncomfortable silence with her.
Perhaps it was in letting my heart fall for a girl who I knew was using me, and desperately trying to buy her the things she wanted, just for the notion that I was at least making her happier.
Or maybe when I was writing letters to a girl, physical letters, obsessing over her every answer and searching in it for some sign of approval, of acceptance, of a willingness to give me a chance. For the brief euphoria of every slightly nicer letter she sent me, and the dreary darkness of her careful distance that she placed me at.
Or was it when, in the depths of my loneliness and rejection, I nearly drowned myself in the lake for seeing two water plants near each other - one seeming to reach for the other, while the other shyly bent away. It resonated in me, and I felt a need to bring them together, to tie them so that they would not part. I went; the current swept beneath me, I swallowed murky waters, but I swam and tied them together. Let them find happiness, even if I do not. I felt the bottom of the river, the sharp cut of rock against my fingers, and then, touched the shore again. I was wet, and my electronics were ruined. Yet it was worth it.
Some girl learned of it, and added my adventure to a character in her webcomic. She thought it was romantic. It was, but it was also stupid of me. And for all of the 'romance' of it, it did not bring me love.
Love wounds, and in some ways, never heals. But the scars that remain give us character.