I asked this too a few chapters ago. Some theories I was given was schizophrenia, PTSD, and psychotic break.
ahhh okay
It’s mainly because of the drugs. In the novel it was stated that Sung-kyung ended up like that due to the drugs and the emotional trauma from his parents’ death, it was the biggest trigger for his breakdown.
The scenes of him taking care of his sick parents, eating spoiled porridge, struggling to survive in Red Mansion, and desperately trying to get money for medicine were all part of his drug induced hallucinations and the lingering effects of withdrawal.
He already had those withdrawal since the beginning (remember how he always write? Well he tried drugs and it wasn't that bad until now) but his parents death and stuff made him use drug more and that why he forget stuff or act like that.
why does mc forget stuff?