"
Taking every breath away,
With all of the mistakes I've made,
From all the letters that I saved,
This is everything I didn't say
He's overcome by all of his emotions as he realizes his mistakes. He can't breathe, thus the line "taking every breath away." The weight of his wrongdoings weighs on him. "From all the letters that I saved," maybe implies that he wrote many letters trying to apologize or get her back but never had the courage to say. "This is everything I didn't say," means that he felt he should have said the things he wrote in this song, but he didn't.