Wasurerarenai

From ShoujoMagic:
A haunting manga that will stay with you always... It is truly an unforgettable story. Nae and Ryou have been going out for almost three years but then something changes... Ryou suddenly breaks up with Nae, leaving her at loss. Nae thought he had a change of heart, but as she found out that it's actually not the case...

This is a very moving story of love and loss.