I'm looking for a romance manga that deals with the pain and angst of unrequited love, especially if it's about someone who you shouldn't fall in love with, like a married person or an older person. It doesn't need to have a happy ending.

I would like it to be seinen, but if you have REALLY good recommendations in other genres, by all means.

See my lists for my tastes. Unscanlated raws are okay.

Two examples that I really liked are Hakuba no Oujisama (not really unrequited, but being in love with someone you shouldn't be) and Fushou no Musuko

Cousin (josei) I felt was very well done. Not sure if that's what you're looking for though.
Definitely Umibe no Onnanoko, if you don't mind mature content.
Pokerface oneshot; contains shounen-ai, but I thought it was a pretty good example.
I haven't read it in a while, but I think Sekine-kun no Koi? I'm pretty sure it doesn't stay unrequited for long though.
Majime na Jikan I remember being pretty good, but unrequited love wasn't really a major theme.
Oh, and shoujo Flowers of Evil perhaps. There's incest--and it's not watered down so warning if you're uncomfortable with that. And the unrequited bit is kind of left ambiguous.

Although its not complete, or completely scanlated for that matter, Natsu no Zenjitsu is looking quite unrequited-love-ish at the latest scanlated chapter so far.