Back in his day, Pablo Neruda was quite the romantic one, who had a plethora of x-wives. It's as if he went through his life with multiple spouses, and with multiple spouses comes multiple fights and long nights of trying to forget that special someone.
In his poem, "Tonight I can write . . ." it's as if Neruda is finally ready to close the chapter on one of his long lost loves, for he has no more interest in her. As the poem begins you get a sort of image as if he still longs for her love and is very much into keeping the relationship. As the poem progresses you finally start to see him turning away. On lines 27-28 he plainly states that he no longer loves her, and in the finaly stanze it is clear the she will no longer cause him any pain nor shall he waste paper writing about her.