|Umm, for this one I'm suggesting, it is a big spoiler, and yes the main girl is indeed in love with the first dude aka B for a while, but yes, she does indeed move on. Here it is anyway, if no one knows already:
Spoiler (mouse over to view)
Tonari no Atashi
Silent war... Love never fades away here, we've got an army of angels... silently bleeding... silently dreaming... not living in Lettuce Garden...