First thing that came to mind was Ultimate Venus.
I think it follows a bit into this plotline..
A guardian falls in love with their charge, but doesn't want to interfere with their charge's normal development as a teenager. So they completely suppress/repress those feelings to play a perfect parental role, only to watch the one they love grow up and marry someone else...

Except the perspective is from the charge's point of view. But the protagonist becomes a rich heiress and is given three candidates for marriage, yet she falls for her caretaker and even though he rejects her (mostly due to his duty..I believe), he says he'll protect her no matter who she marries..

in the first volumes of Ayashi no Ceres
as seen in these two pages:
1, 2

[img]http://img3.lln.crunchyroll.com/i/spire1/d3534fdb6e3e9a20689515e4361f69ea1312335455_full.jpg[/img]
currently reading: Nyotai-ka
please support me

Can't think of any manga where love vs duty as you describe is a main point, but here are the closest I can manage:

Annarasumanara - she used to like magic, but as she grows up, she studies and works hard to earn money. She tries to ignore the male lead, a childish magician, who awakens her old feelings.

Boku to Watashi no Henai Jijou - they break up due to a lack of time to spend together.

Glass Mask - he supports her as an actress, angering her to awaken her passion, even though he loves her and would rather treat her better so she would like him. Age gap and social differences also play a role.

Otoyomegatari - a later arc,

Two characters fall in love, but the father opposes the marriage, and in that culture, the daughter has to obey the father without question.

Ga-Rei - Humans with sixth-senses, spirits; love > duty
Vampire Juujikai - Vampires, duty > love
Akagami no Shirayukihime - Prince and peasant girl, prince ditches duty pretty fast

"People don't change, they just give in."Kubera is the best.同人音楽 (doujin music)

A small sheep dreams - Restricted feelings due to the situation they are both in

edit: also, Do-Re-Mi-Fa-Sol-La-Si-Do is a good example I think 🙂

[img]http://i96.photobucket.com/albums/l184/DancingintheRain_x3/LoveatFirstSightv04-003-1-1.jpg[/img]

ich bin dabei, du bist dabei, wir sind dabei, uns zu verlieren.
ich bin dabei, bist du dabei, sind wir dabei, uns zu verlieren? ♥

manga mood: unexpected 😮
reading: deadman wonderland

You should try
Superior
Superior Cross