Why sungkyung starts forgetting things? Was it because of his drug overdose?

Maybe trauma? Like his body acting on its own, because it wants to forget things that are too harsh for sungkyung . Maybe deep down he must want to forget to, without knowing it.

Probably. He also become reliant to drugs to forget things, I think

Yes! Sung-kyung starts forgetting things primarily due to the effects of drug overdose and withdrawal, combined with emotional trauma. In the novel the biggest factor for the cause was the drugs + his parents death