i want a COMPLETED manga that will make you feel sad and blue throughout...something with a dark/angst theme...here are some mangas ive read and loved that have those elements:

houkago hokenshitsu...this was so unique and original
bitter virgin...realistic and beautiful
thomas no shinzouwhere do i start...i dont know if i'll ever be able to find a touching manga like this one
Kanojotachi no X-Day i loved this!characters were super realistic...only flaw was the shortness of the manga
agehaim talking about the second story only
flowers of evil so beautiful...i loved the obsessive love theme
narration in seventeeneventhough the story is simple...the fact that the female lead's love interest never loved her back throughout the series and only saw her as a sister impressed me...the manga portrayed the pain of unrequited love perfectly
sad love storyone of the few shoujos where i actually didnt feel like ripping the girl's head off
kare kano...oh my god especially the last 4 or 5 volumes...they were breathtaking!!the arima family's past was well written and extremely touching
Deep Love - Ayu no Monogatarii'll never forget this T_T
Watashitachi no Shiawase na Jikanthe most touching manga ive ever read...it was brilliant

now im running out of mangas so pls recommend anything similar to the ones listed above....
oh and there are some mangas which im sure alot of ppl would recommend like : fruits basket and vampire knight.....i just didnt like those...i tried to read them but i just couldnt understand why ppl are going lala over them...so yeah dont recommend those

NO oneshots,smut,hentai,yaoi,yuri or shoujo ai..........shounen ai is fine(only if its a mild case,and by mild case i mean anything NOT like sakura gari...that was just too much)

Maybe Vampire juujikai
Threads of time

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Omae ga Sekai o Kowashitai Nara: a pretty well-known classic, in my opinion, sad and has lots of twists
Made in Heaven: tragic and short original romance
Life isn't focused on romance but it's a pretty good depressing, psychological school life manga, and uh...super dark?
Koizora: depressing and tragic long romance manga about a girl and a guy who later gets diagnosed with cancer
Under the Rose - Haru no Sanka takes place in 19th century England about a huge noble family, it's split up into two main stories, only the second story has romance in it, but overall it's really dark and depressing
Honey Rose is a sequel to Under the Rose, and this one is mainly about family relationships, but once again, has its dark and tells about the tragic ends of certain characters in the prequel
Coelacanth: more psychological/suspense than romance
Kimi ni Shika Kikoenai one shot, gets pretty sad near the end
Hitsuji no Uta is focused around a male protagonist and about "vampires", but this is actually a psychological/angst manga, if any
Eternal Sabbath: not focused on romance, but has the drama/psychological/sad atmosphere
Maou: Juvenile Remix: not a romance manga at all, but worth checking out if you're looking for something psychological, sad, and tragic (well, in the middle)
Watashi no Megane-kun: depressing school romance
Rakuen no Minami: father-daughter relationship
Sunadokei has its share of depressing moments later on
Variante: a sad action manga about a girl who gets attacked by a sort of parasitic monster and becomes its new host

These are nice short pieces that shouldn't be missed:

Not Simple had this lingering feeling of hopelessness throughout the whole manga... but it was beautifully tragic.

Vitamin (SUENOBU Keiko) is angsty and psychological.