Every poem speaks specifically to each reader, but this is how I see the meaning of the poem.
Yeats (or the speaker) is writing about "writing." He recalls a time when his "darling" could not understand why he has chosen to be a writer in a world which he sees so often as "this blind bitter land."
He remembers, too, growing "weary" of the world, especially the sun, which all life depends on. Eventually, however, the speaker comes to his senses and recalls the best work he has done in trying to clarify his view of the world.
He has come to the point that every year he reassures himself that his "darling" understands what his intent is in writing because it makes him strong and he has a way with words: "...words obey my call..."
Silently, on the side, he wonders if she really has come to an understanding. If she has not, he wonders if had he had that knowledge, would it have stopped him—what would have been lost? ("...shaken from the sieve...") Had he listened and changed his course, he realizes that he might have given up ("...thrown poor words away..."), and learned to be satisfied with a life without words. The sense here is that had he done so, it would have been no life at all.