“All who strive to live for something beyond mere selfish aims find their capacities for doing good very inadequate to their aspirations. They do so much less than they want to do, and so much less than they, at the outset, expected to do, that their lives, viewed retrospectively, inevitably look like failure.”

Source: Runaway

Source: So Good They Can't Ignore You: Why Skills Trump Passion in the Quest for Work You Love

"Andrea del Sarto", line 70
"Less is more" is often misattributed to architects Buckminster Fuller or Ludwig Mies van der Rohe. It is something of a motto for minimalist philosophy. It was used in 1774 by Christoph Martin Wieland.
Men and Women (1855)
Context: I do what many dream of, all their lives,
— Dream? strive to do, and agonize to do,
And fail in doing. I could count twenty such
On twice your fingers, and not leave this town,
Who strive — you don't know how the others strive
To paint a little thing like that you smeared
Carelessly passing with your robes afloat —
Yet do much less, so much less, Someone says,
(I know his name, no matter) — so much less!
Well, less is more, Lucrezia: I am judged.
There burns a truer light of God in them,
In their vexed beating stuffed and stopped-up brain,
Heart, or whate'er else, than goes on to prompt
This low-pulsed forthright craftsman's hand of mine.

"The Expanding Mental Universe", Saturday Evening Post (July 1959)
1950s

Debate with Barry Goldwater, University of Arizona campus, Tucson, Arizona, November 1961

Knowledge is Power
The Note-Books of Samuel Butler (1912), Part VII - On the Making of Music, Pictures, and Books

Pelagea Vlasova in Scene 10
The Mother (1930)
Variant: Don't be afraid of death so much as an inadequate life.
Source: Jewish Wife and Other Short Plays: Includes: In Search of Justice; Informer; Elephant Calf; Measures Taken; Exception and the Rule; Salzburg Dance of Death

Source: The Way of All Flesh (1903), Ch. 5