Experience iz a grindstun, and it iz lucky for us if we kan git brightened by it, not ground.

[Experience is a grindstone, and it is lucky for us if we can get brightened by it, not ground.

Josh Billings (1818-1885) American humorist, aphorist [pseud. of Henry Wheeler Shaw]

Everybody’s Friend, Or; Josh Billing’s Encyclopedia and Proverbial Philosophy of Wit and Humor, ch. 130 “Affurisms: Puddin & Milk” (1874)
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This aphorism was transformed / paraphrased in the early 1920s into something a bit more inspirational, first (it appears) in Forbes (1922-10-14), then in similar form in other periodicals such as The Beaver (1924-03) and Wood Construction (1924-09-15). The new form:

Life is a grindstone, and whether it grinds a man down or polishes him up, depen's on the stuff he's made of.

In an earlier pass of Billings quotations, I did up a meme, unknowingly based on that later phrasing: