“The greatest of all mistakes is to do nothing because you think you can only do a little.”
Related quotes
Maitland Tapes-interview with Prof. Hugh Maitland 1970 L. S. Lowry - A Biography by Shelley Rhode Lowry Press 1999 ISBN 9781902970011.
Maitland Tapes

“They can because they think they can.”
Aeneid (29–19 BC), Book V
“They can because they think they can.”
Source: Translations, The Aeneid of Virgil (1866), Book V, p. 153

“They can because they think they can.”
Possunt, quia posse videntur.
Source: Aeneid (29–19 BC), Book V, Line 231 (tr. John Conington)

TV Series and Specials (Includes DVDs), Mind Control (1999–2000) or Inside Your Mind on DVD

“It is the greatest of all mistakes to do nothing because you can only do little.”
Lecture XIX : On the Conduct of the Understanding, Part II
Elementary Sketches of Moral Philosophy (1849)

Interview http://www.nytimes.com/2015/11/22/world/asia/kim-young-sam-former-president-of-south-korea-dies-at-87.html?_r=0 (2009)
2000s, 2009

“Laila has moved on. Because in the end she knows that’s all she can do. That and hope.”
Source: A Thousand Splendid Suns