A poet who believes in the worthiness of his or her work
may, in the end, turn out to have been wrong in that belief—
but a poet who doubts the merit of their own work will almost always
turn out to be right.
(Maybe it's not just poets this applies to.)
This is in line with the terror expressed by (presumably) Delmore Schwartz, as it appears in Saul Bellow's Humboldt's Gift (1975), page 120, where the narrator summarizes a frequent lament of Humboldt (a character based on Schwartz) regarding the "profession" of poetry:
[Humboldt always said] that poetry was one of the frantic professions in which success depends on the opinion you hold of yourself. Think well of yourself, and you win. Lose self-esteem, and you're finished. For this reason a persecution complex develops, because people who don't think well of you are killing you. Knowing this, or sensing it, critics and intellectuals had you. Like it or not you were dragged into a power struggle.
Whether this was said by Schwartz himself, or concocted by Bellow for his character Humboldt to voice,
I think it's one of the truest things I've ever read about the frantic experience of being a poet—