Self-hate
Self-hatred is like the rot on a log that has been floating in a river for far too long and is close to breaking apart and separating forever. It's like a termite that eats at an old house, chewing through a beam that once destroyed will bring the entire house collapsing down. It's a disease, a monster, a creature that never dies but only grows and grows and grows until you can't see a single thing about yourself besides flaws and ugly traits.
Self-hatred festers when left unchecked, but in its duality also gnaws at you when you do your best to ignore it. It's a jail-cell made of mold and tiny insects that bite at your toes and nip at your skin until everything about you is ugly, and you don't have a single redeeming quality. Through that jail-cell people may try to break you out, may try to show you that your cell isn't really a cell at all, but an open room in which one can flourish. But you know the truth, the truth that your hatred towards yourself has shown you; that there is no hope for you anymore, no matter who tries to convince you otherwise.
You are ugly. You are stupid. You are undesirable and selfish and a waste of space. What good are you alive if the world would be better off with you dead?
And that, my friend, only touches the surface of what self-hate is.