Lost quotations

Drove myself crazy a year or so ago trying to track down a quotation I thought was from Aquinas. (Turned out to be the great Josef Pieper.) So far today I've spent over an hour trying to track down some lines that I am convinced were from "The Elements of Drawing" by John Ruskin. But not even word-searching complete online copies of the text (PD, of course) has turned up anything. So I've either misremembered the source or the lines or (most likely) both.

The gist is something like this: "To draw the best picture you are capable of, stop when you feel the slightest difficulty. But if you want to improve, push on when you feel difficulty. Go on to trying to correct the drawing until you've erased it so full of holes that you can't continue. Today's drawing will be ruined, but the drawing you do tomorrow will be better than today's best."

A simple proscription with interesting implications -- suggesting, for instance, the long term dangers of doing creative work under contract or for hire, situations in which one would be all but morally obliged to settle for "today's best."