It's about how he can't stop thinking about her "I smelled you on my shirt today" but he knows that being near to her will be worse, in the long run. "Of course the hardest part is letting go/ but you've got to, or you know/ you'll end up waiting by the/ phone..." So he has to do the sucky thing and move on, to stop seeing her and separate himself from his past: "Tell you again that I'm fine/ then we go about our separate lives"
It's about how he can't stop thinking about her "I smelled you on my shirt today" but he knows that being near to her will be worse, in the long run. "Of course the hardest part is letting go/ but you've got to, or you know/ you'll end up waiting by the/ phone..." So he has to do the sucky thing and move on, to stop seeing her and separate himself from his past: "Tell you again that I'm fine/ then we go about our separate lives"