Personally I think that this song is about a guy who's already broken up with the girl.
She fights and fights for him, but he just won't come back...he's already moved on, and she still loves him.
He's already hurt her by breaking her heart, but when she sees him with the girl he likes, and he's smiling at her, the way he used to when they were together...she knows he has feelings for her, and gets her heart broken all over again.
Personally I think that this song is about a guy who's already broken up with the girl. She fights and fights for him, but he just won't come back...he's already moved on, and she still loves him. He's already hurt her by breaking her heart, but when she sees him with the girl he likes, and he's smiling at her, the way he used to when they were together...she knows he has feelings for her, and gets her heart broken all over again.