The work of a young writer--Werther is the classic example--is sometimes a therapeutic ac. He finds himself obsessed by certain ways of feeling and thinking of which his instinct tells him he must be rid before he can discover his authentic interests and sympathies, and the only way by which he can be rid of them forever is by surrendering to them. Once he has done this, he has developed the necessary antibodies which will make him immune for the rest of his life. As a rule, the disease is some spiritual malaise of his generation. If so, he may, as Goethe did, find himself in an embarrassing situation. What he wrote in order to exorcise certain feelings is enthusiastically welcomed by his contemporaries because it expressed just what they feel but, unlike him, they are perfectly happy to feel in this way; for the moment they regard him as their spokesman. Time passes. Having gotten the poison out of his system, the writer turns to his true interests which are not, and never were, those of his early admirers, who now pursue him with cries of "Traitor!"
That Auden is fine with an aphorism is beyond (reasonable) dispute (there are always wags and cranks, after all). His vision of adulthood has something very compelling to it: sometimes it takes the form of "one must first learn to be oneself, then must learn to be not oneself;" here it takes the opposite form--it is only by having the disease fully that we can pass beyond it. The point, so far as I can see, is that a person, will it or not, is always in the process of changing. Some of those things will be more like who that person really is, some will be less--though it will not always be clear to the person involved which is which, sometimes not clear until long after. That change will happen is a given: the choice is whether we let it happen to us or take control of it; let ourselves be directed to ends by outside forces or choose our own.
But, and this is key, the perspective on which to judge any particular moment is not the one we find ourselves in now. One doesn't judge the worth of Werther by recognizing Goethe abandoned, perhaps never really held, its point of view (there are other questions that have this comparative angle, but they come later).