reason: i had some things on the floor. i told my mom i'd take care of it later. my dad started yelling at me about how this was my choice or whatever

result: i tried stabbing my left arm with a blunt pencil 8 times

Not to presume your situation, but: In cases where parents don’t allow/accept protest and confrontation, the feelings that were meant to be expressed outwards get turned inwards instead. Like branches of a tree, they’ll grow in one direction or another. It’s called internalisation (as a result of self-suppression).

I used to work in this area and I hope you can find someone skilled and compassionate to talk with because this is workable.