I think it has something to do with her decision to become a vampire. Some of the song fits with that "Now when I caught myself, I had to stop myself,.
I'm saying something that I should have never thought.
Now when I caught myself, I had to stop myself,.
From saying something that I should have never thought of you. " I'm pretty sure Edward was disturbed by the idea of turning her. Been awhile since I've read Twilight though.
The movie is covering the first book right? The breakup happens in the second book so don't think the song has anything to do with that.