looking for manga where a character shows obvious interest in the main character, and the main character is aware of that and exploits the other character's feelings selfishly
and this other character puts up with this until he/she is fed up and leaves and it is at this point that the main character finally understands that they're in love with the other character but it is too late to do anything about it

OR

just any manga that ends with the main character harboring unrequited love for another character

examples:
Marie no Kanaderu Ongaku <3 <3
a girl by the sea
Kabocha to Mayonnaise
5 cm per second
sayonara midori-chan
Mars no Kiss
Blue
almost all the oneshots in Itaitashii Love

Hachimitsu to Clover. It's a great manga, I really recommend it. There's unrequited love but no exploitation.

Dirty Work, perhaps? Although the main character is the one being used...

Also, are you looking specifically for love that is not reciprocated in the end, or would an unconsummated love work as well? (What I mean by that is that the ending makes it clear that the love will never be consummated in the future, rather than merely stopping before that point.)

Edit: I was thinking something along the lines of the ending of Ryuguden, which doesn't seem to fit what you were looking for. So never mind...

Quote from hahhah42

Dirty Work, perhaps? Although the main character is the one being used...

Also, are you looking specifically for love that is not reciprocated in the end, or would an unconsummated love work as well? (What I mean by that is that the ending makes it clear that the love will never be consummated in ...

I'm not sure I understand what you mean
I'm looking for love that is not reciprocated but if by unconsummated you mean feelings are also one sided then that's fine too...I don't care if characters are physically involved as long as it's clear that only the main character is in love

If you don't mind shounen ai, Croquis chapters 5 and 7

Datte, Kimi ga Warau kara.- second chapter
Shite, Okeba Yokatta Koukai - chapters 5-6
Boku no Mei
Chouchou Kumo -first chapter

These stories all end with unrequited love.

Sakamichi no Apollon maybe.

Koi Koinegau, a metaphorical one-shot where the guy has unrequited love.

I'm in idol hell T.T