http://www.mangago.zone/read-manga/koukai_no_nai_jinsei_wo/
http://www.mangago.zone/read-manga/time_letter/
^^ literally anything by this author is good but sad
http://www.mangago.zone/read-manga/bradherley_no_basha/
^^ this one was just dark and depressing
i didn't feel depressed tbh i just felt completely empty after i read it
http://www.mangago.zone/read-manga/my_childhood_friend_was_a_god/
try reading these stories by this author
http://www.mangago.zone/r/l_search/?name=Avogado6
What's the most depressing one-shot manga you've read
Idc what genre it is but I want it to make me feel pain for months
just like banana fish did