Guys lol it's not any of that. Dohyun lost part of his memory when he almost slipped and died during a delivery. http://www.mangago.me/read-manga/warehouse/uu/warehouse-chapter-1/34/ He says it himself on the bottom right. That's why everything was fuzzy and he has a hard time remembering stuff.
Didn't the author say that he just lives in the present?
http://www.mangago.me/read-manga/warehouse/uu/warehouse-chapter-36.6/29/

how in the word did he forget Sungho that genuinely just doesn't make sense