Pretty much i'm wondering if there is any series like
Watashitachi no Shiawase na Jikan

I really loved it.
It has to be romance.
and good art.
I don't like yuri or hentai. and no echii. (i don't know if i spelled that right -.-'')

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Hoshi no Koe, by the same author, is similar.

Other darkish+romance series:

Shoujo/Josei:
Please Save My Earth
Flower of Evil
Mars
River's Edge
Kare Kano

Shounen/Seinen:
Lament of the Lamb
D-ASH
Onani Master Kurosawa
Kiseijuu

Try Uzumaki. I don't know if it's what you're looking for, but it is a very creepy manga about a girl her boyfriend who live in this town that gets cursed. all these strange horrible things start happening-and they all have to do with spirals. people go insane, become less than human....it's just creepy. And the ending is very haunting. V. interesting manga.

Recommended dark romances:

Dokuhime

and

Savage Garden

School Days

I've never read it but its starts with romance, gets dark, art looks fine.

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二息歩行

I suggest Immortal Rain, it is slight shounen-ish (lots of fight/action) but primarily shoujo. It has realistic romance and a deep, original and haunting storyline. Rain (male lead character) and the antagonist's history (which is revealed throughout the course of the series, not overwhelming you all at once in the first chapter with a typical mediocre plot) is entirely unique and really stays with you.

I'd second Immortal Rain.

I loved Watashitachi no Shiawase na Jikan! For similar emotional depth and excellent characters, the author's other works are a safe bet, though sadly, not as "dark".

I'd also recommend The Embalmer. It's extremely dark (the main male lead is an embalmer, so...) and the romance eventually takes lead importance. It's really good.

Except for the lack of romance, Dolls fits the bill. It's just an awful lot of bishies, though. There's hardly any romance to speak of. It's an excellent series, though.

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I'd recommend Berserk.

It doesn't get much darker.

Edit: Sorry, I see you specified no ecchi. There is ecchi in Berserk.

I'd second and third Berserk. But...the romance does not pick up until later volumes...and it is still ongoing.

Elfen Lied might be something else you could look into.