“Do what is beyond your strength even should you fail sometimes.”
Cork address (1885)
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Charles Stewart Parnell 11
Irish politician 1846–1891Related quotes

“Make sure you are doing what God wants you to do—then do it with all your strength.”
This saying is not found in any source before 2010, when it was posted by The Ignorant Fisherman at Free Republic http://www.freerepublic.com/focus/bloggers/2451682/posts on 15 February. The language is not that of Washington or his time.
Misattributed, Spurious attributions

“Sometimes doing your best is not good enough. Sometimes you must do what is required.”
Source: Drenai series, The King Beyond the Gate, Ch. 23

As quoted in The ArtSlut's Guide to Makin' It — As a Visual Artist (2007) by Barb Benson
Context: You have to dream, you have to have a vision, and you have to set a goal for yourself that might even scare you a little because sometimes that seems far beyond your reach. Then I think you have to develop a kind of resistance to rejection, and to the disappointments that are sure to come your way.

Speech in Edinburgh (25 November 1879), quoted in W. E. Gladstone, Midlothian Speeches 1879 (Leicester University Press, 1971), p. 46.
1870s
Source: The Girls' Guide to Hunting and Fishing