I think this is the best song on the album!
The letter he is talking about is one that his (now ex I would guess) girlfriend has left for him because she can't tell him to his face that she doesn't want to be with him. And even though he knows what it says he still can't open the letter, because if he does it will draw a line under the relationship and confirm that it is over. If he doesn't open it there is still a chance in his mind that he hasn't lost her.
I think this is the best song on the album! The letter he is talking about is one that his (now ex I would guess) girlfriend has left for him because she can't tell him to his face that she doesn't want to be with him. And even though he knows what it says he still can't open the letter, because if he does it will draw a line under the relationship and confirm that it is over. If he doesn't open it there is still a chance in his mind that he hasn't lost her.