Using my fingernails, I scratched at my
Using my fingernails, I scratched at my scalp until it bled, making small wounds that didn't heal for months, due to my scratching. I've long since stopped, but now I have small, round places on my scalp where hair no longer grows. One can't see them unless I part my hair perfectly down the middle.
Even though nobody knows, it's the shame that I ever did something that obsessively painful to myself that makes me feel ugly, even today.