I want the saddest manga you've read. I feel like crap rn so what better way than just rea...

First thing I can think off the top of my head is
" Smells like Green Spirit" which I recommend to anyone who hasn't read it

The Specific Heat Capacity of Love definitely fits the description

http://www.mangago.me/read-manga/the_specific_heat_capacity_of_love/

Also, Till Dawn is a very good one

http://www.mangago.me/read-manga/till_dawn/

Sakura gari is the saddest thing I've ever read. I cry like a baby every time I read it

"Girl may kill" is sad. (Sorry I don't read a lot of sad stuff.)

http://www.mangago.me/read-manga/watashitachi_no_shiawase_na_jikan/
http://www.mangago.me/read-manga/bokurano/
http://www.mangago.me/read-manga/a_lollipop_or_a_bullet/

Yaoi, life senjou no bokura. I bet you will cry if you like yaoi stories. It has a happy ending but it is so saddd

Elfen Lied. A bit gory, but it is just heartbreaking

Natsume Yuujinchou isn't super ultra depressing but it made me cry like hell. (literally, every chapter and episode)

http://www.mangago.me/read-manga/deep_love_pao_no_monogatari/
This and the other deep love stories are a whole other level of sad. Heartbreaking enough to hurt your very soul

http://www.mangago.me/read-manga/koizora/

oyasumi punpun. it bored me to death though

http://www.mangago.me/read-manga/oyasumi_punpun/