Dissociation, Isolation, Self-harm, Suicide
(not a native, there might be mistakes in what I write)
The book I'm looking for must have all of those in the title, I'm looking for a book which brings the sense of total despair and so on
The MC is stuck in his/her own head and the stream of consciousness is weird, distant, meaningless or terribly bizarre.
The only book i read so far that matches these aspects is "I'm Thinking Of Ending Things" by Ian Reid. The atmosphere and lore are spot-on. And look, i know it's not "extreme horror", it's just an example. I'm looking mostly for psychological horror, but self-harm gory stuff will make it even better. But the psychological part must be real deep and stuff, the book shouldn't be just a gore-fest with a suicidal plot. So feel free to suggest.
The only other criteria must be that the book should have a lot of stream of consciousness and ideally be first-person.