You searched for: “sectantem levia nervi deficiunt animique
Brevis esse laboro, obscurus fio; sectantem levia. Nervi deficiunt animique. (Latin proverb)
Translation: "When I try to be brief, I become obscure. Aiming at smoothness, I fail in force and fire."

From Ars Poetica, by Quintus Horatius Flaccus (Horace) who was instructing writers that it may be difficult to achieve brevity without sacrificing clarity.

Not that the story need be long, but it will take a long while to make it short."

—Henry David Thoreau (1817-1862) U.S. naturalist and author