Guy dumps girl after she cheated on him with another guy.
"It started the day when I left you" (The day I cheated on you)
She still loves him so now she feels sorry and apologizes for cheating.
"There was no reason. Just a foolish beat of my heart"
But it is too late. He no longer has feelings for her.
"Oh, the look in your eyes Just left me beside myself without your heart"
She can no longer fall in love with anyone else because she only loved him.
"I could never love again now that we're apart"
[Edit: For a long time I used to be confused about the lyrics of this song:
Why is the girl sad about the guy leaving her, when it was she who left him.
It only make sense when you interpret her "leaving" as her "cheating" and him leaving ]