I've read a lot sad mangas but these are the only ones i can remember
https://www.mangago.me/read-manga/hate_no_kouya_de_vacances_wo/
https://www.mangago.me/read-manga/asagao_wa_yoru_kara_saku/
I strongly suggest you to NOT read/look at the genre or tags as this sometimes spoil the story (well for me it affects the whole point of me reading it)
http://www.mangago.me/read-manga/1866/
https://www.mangago.me/read-manga/sea_foam/
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This is oneshot ...but this got me depressed for whole week ╥﹏╥
I can’t decide, so imma drop u a list of all the things i have cried about ╥﹏╥
https://www.mangago.me/home/mangalist/1061343/?filter=&page=1
Everyone pls drop ur saddest manga u've read I just want to cry myself to sleep:))