What would it be, to not see categories as real?

I expect it would be noticing that I treat X as though it were importantly similar to Y, even though X is (it seems to me) nothing at all like Y.

This happened to me a lot while I was dealing with post-stroke PTSD... I would react to things in ways that made no sense to me at all, think about it for a while, and eventually conclude that I was treating those things as importantly equivalent to aspects of stroke-related trauma, even though they didn't seem to me to be importantly equivalent at all.

Our minds are not internally consistent.

Agreed about the rest of this, though. "Aaaa! I'm stuck inside this dark, damp skull!" just isn't the sort of thing brains are wired to experience.