“Endurance is mind over matter.”

Variant: It's called mind over matter. If we don't mind, it doesn't matter.
Source: Room (novel) (2010)

Source: Callahan's Crosstime Saloon

Source: The Picture of Dorian Gray

Bernard Baruch in response to a question by Igor Cassini as to how he handled the seating arrangements at his dinner parties, as quoted in Shake Well Before Using: A New Collection of Impressions and Anecdotes Mostly Humorous (1948) by Bennett Cerf, p. 249; the full response was "I never bother about that. Those who matter don't mind, and those who mind don't matter." This anecdote is also quoted online at Chiasmus.com http://www.chiasmus.com/archive/msg00241.html. It has also become part of a larger expression, which has been commonly attributed to Dr. Seuss, even in print, but without citation of a specific work: "Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind."
Misattributed
Variant: Be who you are and say what you feel because those who mind don't matter and those who matter don't mind.

Often quoted response to Igor Cassini, a popular society columnist for the New York Journal American, when asked how he handled the seating arrangements for all those who attended his dinner parties, as quoted in Shake Well Before Using: A New Collection of Impressions and Anecdotes Mostly Humorous (1948) by Bennett Cerf, p. 249; the full response was "I never bother about that. Those who matter don't mind, and those who mind don't matter." This anecdote is also Chiasmus and has also become part of a larger expression, which has been commonly attributed to Dr. Seuss, even in print, but without citation of a specific work : "Be who you are and say what you feel, because those who mind don't matter and those who matter don't mind."

Source: The Powerbook (2000)

Source: Essays in Persuasion (1931), The End of Laissez-faire (1926), Ch. 3