Firmness in decision is often merely a form of stupidity. It indicates an inability to think the same thing out twice.

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

A Little Book in C Major, ch. 5, § 30 (1916)
    (Source)

Variant:

FIRMNESS: A form of stupidity: proof of an inability to think the same thing out twice.
[A Book of Burlesques, "The Jazz Webster" (1924)]