Fairly simple situation: a girl (or boy) manages to gain a relationship with a boy (or girl) she/he likes, but once they're together she finds herself thinking, talking, even wanting someone else, leading to her ultimately breaking off the relationship and trying to make it work with her true love. I know this's happened a fair number of times, but the first that comes to mind (as it's the most recent example I've read) is Shishunki no Iron Maiden, in which the main heroine is demonstrated to slowly fall in love with the main character (literally, there are physical signs when she starts to develop feelings for him and while they grow) while trying to get together with her crush; eventually she manages to confess to her crush, and when he takes her on a romantic date, all she can think about is the main character. I won't spoil what happens after, as it's a little involved and close to the climax.