“Ignorance more frequently begets confidence than does knowledge.” --Charles Darwin
“Our doubts are traitors and make us lose the good we oft might win, by fearing to attempt” --[Measure for Measure, I-IV] --Shakespeare
“Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe.” --Albert Einstein
"Grant me the serenity to accept the things I cannot change, the courage to change the things I can, and the wisdom to know the difference.” --Reinhold Niebuhr? (The Serenity Prayer)
“For my part I know nothing with any certainty, but the sight of the stars makes me dream.” --Vincent van Gogh
“Success is not final, failure is not fatal: it is the courage to continue that counts.” --Churchill
There are also plenty of good ones at this thread:
https://www.mobileread.com/forums/showthread.php?t=82285