Not so much ago I noticed I'm addicted to psychological manga where the protagonist is bullied by some "strange" person. And when I say strange I mean someone who looks like they have no feelings at all, as if they weren't human, doing such things as blackmailing the protagonist in order to make them do something illegal/heart-breaking, and furthermore, they truly psychologically torture the protagonist, and that suffering is seen when they have mental debates.
To have an idea of what I'm looking for, take as an example the iconic Aku no Hana or Scumbag Loser. I don't mind romance, gore, horror or whatever. I particularly enjoy when they take place at a school, though (not trying to be picky).