Every failure teaches a man something. For example, that he will probably fail again next time.

H. L. Mencken (1880-1956) American writer and journalist [Henry Lewis Mencken]

A Little Book in C Major, ch. 2, § 24 (1916)
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Variants:

EXPERIENCE. A series of failures. Every failure teaches a man something, to wit, that he will probably fail again next time.
A Book of Burlesques, "The Jazz Webster" (1924)

Every failure teaches a man something, to wit, that he will probably fail again next time.
Chrestomathy, ch. 30 "Sententiae" (1949)