HE WHO HESITATES IS LOST - "Swift and resolute action leads to success; self-doubt is a prelude to disaster. The proverb goes back to 'Cato' (1713) by English essayist and poet Joseph Addison. First attested in the United States in 'The Autocrat of the Breakfast Table" (1858) by Oliver Wendell Holmes." From "Random House Dictionary of Popular Proverbs and Sayings" by Gregory Y. Titelman (Random House, New York, 1996).
Another source says early uses of the proverb related specifically to women. "When love once pleads admission to our hearts, the woman that deliberates is lost," wrote Addison. From the "Concise Oxford Dictionary of Proverbs" by John Simpson and Jennifer Speake (Oxford University Press, Oxford and New York, 1998).
"A familiar saying in the twentieth century, "He who hesitates." was quoted by American novelist Edgar Rice Burroughs in 'Gods of Mars' (1912), Eugene O'Neill in 'Beyond the Horizon' (1920), and English novelist Wyndham Lewis in 'The Apes of the God' (1930)." From "Wise Words and Wives' Tales: The Origins, Meanings and Time-Honored Wisdom of Proverbs and Folk Sayings Olde and New" by Stuart Flexner and Doris Flexner (Avon Books, New York, 1993).