I would say Jealousy Incarnate would fit this because she does fall for another guy while being totally oblivious to the fact that the main lead loves her.
*Spoiler (Maybe)*
And what I thought was the kicker is that the main lead realizes that he's pretty much handed her over on a silver platter to the other guy at the same time he realizes he loves her. So he tries his hardest to stand by and watch her be happy--with his best friend on top of it all--and when he gives up the stupid nobility act the show goes all kinds of crazy.
