Quotes about difficulty
page 7

Letter to George Washington (9 October 1776)

pp. 57–58 https://babel.hathitrust.org/cgi/pt?id=nyp.33433068235500;view=1up;seq=87
Ecce Homo (1866)

Quote of Camille Pissarro, Paris, 4 May 1883, in a letter to his son Lucien; from Camille Pissarro - Letters to His Son Lucien ed. John Rewald, with assistance of Lucien Pissarro; from the unpublished French letters; transl. Lionel Abel; Pantheon Books Inc. New York, second edition, 1943, pp. 29-30
his comment after having seen his own painting-show at Durand-Ruel 's gallery in Paris, May 1883
1880's

full of ingenious difficulties [= translation of Greek art historian Nicos Hadjinicolau] /
full of deceptive difficulties [= translation of Spanish art historians Xavier de Salas and Fernando María]
Quote of El Greco, as cited in 'Hand-written Note Shows El Greco Defending Byzantine Style In Face Of Western Art', Dec. 2008 https://www.sciencedaily.com/releases/2008/12/081218132252.htm
the different translation by Nicos Hadjinicolau leads him to the conclusion that El Greco was defending Byzantine art; which is rejected by Fernando María

InsideOut Hudson Valley, January/February 2009

1920s, Second State of the Union Address (1924)

The Cornerstone Speech (1861)

Conclusion : The Moral of this Examination
A Perplexed Philosopher (1892)

1920s, Toleration and Liberalism (1925)

Source: The Theory of Electrons and Its Applications to the Phenomena of Light and Radiant Heat (1916), Ch. V Optical Phenomena in Moving Bodies.

Source: The Case of Mr. Richard Arkwright and Co., 1781, p. 23-24

Source: How to Pay for the War (1940), Ch. 3 : Our Output Capacity and the National Income

Speech in the Reichstag (19 February 1918), quoted in W. M. Knight-Patterson, Germany. From Defeat to Conquest 1913-1933 (London: George Allen and Unwin, 1945), pp. 149-150.
1910s

"Ration before the University of Cambridge on being elected Lucasian Professor of Mathematics," (1660), reported in: Mathematical Lectures, (1734), p. 28

On Coalition Government (1945)

pg.50, 1860 edition
Quotes from secondary sources, Smooth Stones Taken From Ancient Brooks, 1860

Descriptio Globi Intellectualis (1653, written ca. 1612) Ch. 6, as quoted in "Description of the Intellectual Globe," The Works of Francis Bacon (1889) pp. 517-518, https://books.google.com/books?id=lsILAAAAIAAJ&pg=PA517 Vol. 4, ed. James Spedding, Robert Leslie Ellis, Douglas Denon Heath.

Conjectures and Refutations: The Growth of Scientific Knowledge (1963)

Letter to Tench Coxe (20 March 1820), Montpelier https://books.google.com/books?id=EgpFAQAAMAAJ&pg=PR20&dq=%22portentous+evil%22+%22Madison%22&hl=en&sa=X&ved=0CCMQ6AEwAWoVChMIzqj-_8bOxwIVBnc-Ch365g4C#v=onepage&q=%22portentous%20evil%22%20%22Madison%22&f=false
1820s

Source: The Pivot of Civilization, 1922, Chapter 8, "Dangers of Cradle Competition" (also quoted in Charles Valenza, "Was Margaret Sanger a Racist?" Family Planning Perspectives, January-February 1985, page 44.)

1860s, Letter to Abraham Lincoln (1863)

[Swami Tapasyananda, Swami Nikhilananda, Sri Sarada Devi, the Holy Mother; Life and Conversations, 370]

Adams as misquoted by David Barton, in "The Dream of Dr. Benjamin Rush & God's Hand in Reconciling John Adams and Thomas Jefferson" in WallBuilders (June 2008) http://www.wallbuilders.com/LIBissuesArticles.asp?id=10152; omitting many words, giving a very misleading impression that Adams (who did not believe in the Christian Trinity) is endorsing the viewpoint that a government must be administered by the Holy Ghost to be legitimate. Barton went on to use another version, substituting some of Adams' words with false ones:
Misattributed

Preface
The Substitution of Similars, The True Principles of Reasoning (1869)

(version in original Dutch / origineel citaat van Suze Robertson:) Moeilijkheden met mijn leerlingen [o.a. op de Rotterdamse H.B.S. - waar ze met lesgeven begon - van 1876 tot 1882] heb ik nooit gehad, want ik was voorbereid op hun streken, omdat ik gelukkig zelf dikwijls ondeugend was geweest. Wat hadden we op de Haagsche Academie vaak 'n ontzettende pret gemaakt!. .Dus had ik mijn eigen ervaring op dit gebied nog frisch in 't geheugen.
Suze was teaching first in Rotterdam at the Dutch High School, from 1876 to 1882, and afterwards one year in Amsterdam, 1883; then she stopped teaching
Source: 1900 - 1922, Onder de Menschen: Suze Robertson' (1912), p. 30

Miscellaneous Works and Correspondence (1832), To Mr. Cleveland Secretary of the Admiralty (April 14, 1760)

“Translated: "to the stars through difficulties."”
State motto of Kansas, reported to have been devised by Ingalls.
Attributed

"Galtieri muses on what-if's of Falkland war" http://www.nytimes.com/1982/09/16/world/galtieri-muses-on-what-if-s-of-falkland-war.html, The New York Times (September 16, 1982)

Source: L’Expérience Intérieure (1943), p. 12

pg: 12
The Worlds of Herman Kahn: the intuitive science of thermonuclear war.

The Education of Henry Adams (1907)

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 194.
Source: Sylvia cartoon strip, p. 154
Mount Analogue (1952)

In a letter to Burke Aaron Hinsdale (1 January 1867); quoted in The Life of Gen. James A. Garfield (1880) by Jonas Mills Bundy, p. 77
1860s

The Tree That Fell To The West: Autobiography of a Sufi (2003)

1990s, Speech at Ohio Wesleyan University (1997)

Letter to Cassandra (1801-05-21) [Letters of Jane Austen -- Brabourne Edition]
Letters

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 138.

No. 255 (22 December 1711).
The Spectator (1711–1714)

“Speech to the First All-Russia Congress of Workers in Education and Socialist Culture” (30 July 1919) http://www.marxists.org/archive/lenin/works/1919/aug/05.htm Collected Works, Vol. 29, p. 535.
1910s

Source: On the Study and Difficulties of Mathematics (1831), Chapter I. Introductory Remarks on the Nature and Objects of Mathematics.

(original Dutch: citaat van Willem Roelofs, in het Nederlands:) [een aquarel] is niettegenstaanden alle moeite, en misschien wel omdàt zij moeite heeft gekost, minder frisch en wat zwaar geworden – Ik heb zelf lang in beraad gestaan of zij goed genoeg was om te zenden [naar Utrecht]
In a letter to P. verLoren van Themaat, 23 Nov. 1866; in Haagsch Gemeentearchief / Municipal Archive of The Hague
1860's

Quoted in “Collected works of Periyar E.V.R.” p. 511.
Rationalism

Of Princes.
Political, Moral, and Miscellaneous Reflections (1750), Political Thoughts and Reflections

Journal entry, December 31, 1877

Speech https://api.parliament.uk/historic-hansard/commons/1933/apr/13/adjournment-easter-1#column_2790 in the House of Commons (13 April 1933)
The 1930s
Berners-Lee, Cailliau, Luotonen, Nielsen and Secret, 1994
First paragraph
Convergence to the Information Highway (1996)

No. 156
Characteristics, in the manner of Rochefoucauld's Maxims (1823)
Methodical Realism

“We have here a question of difficulty, analogous to the question of nominalism and realism.”
The Law of Mind (1892)

The Differential and Integral Calculus (1836)
Karl E. Weick, " Drop your tools : An allegory for organizational studies http://www19.homepage.villanova.edu/gregory.gull/MBA8510.html/DropTools_Weick.htm." Administrative Science Quarterly. v41 n2. Jun 1996. p. 301-31
1980s-1990s
?
The Journey Home: Autobiography of an American Swami (Tulsi Books, 2010)

This quote is commonly attributed to Churchill, but appears in the "Red Herrings: False Attributions" appendix of Churchill by Himself : The Definitive Collection of Quotations (2008) by Richard Langworth, without citation as to where it originates.
In American Character, a 1905 address by Brander Matthews, a similar quotation is attributed to L. P. Jacks ( link http://hdl.handle.net/2027/mdp.39015059451156?urlappend=%3Bseq=238).
""Our civilization is a perilous adventure for an uncertain prize... Human society is not a constructed thing but a human organization... We are adopting a false method of reform when we begin by operations that weaken society, either morally or materially, by lower its vitality, by plunging it into gloom and despair about itself, by inducing the atmosphere of the sick-room, and then when its courage and resources are at a low ebb, expecting it to perform some mighty feat of self-reformation... Social despair or bitterness does not get us anywhere... Low spirits are an intellectual luxury. An optimist is one who sees an opportunity in every difficulty. A pessimist is one who sees a difficulty in every opportunity... The conquest of great difficulties is the glory of human nature." L. P. Jacks, quoted in American character, by Brander Matthews, 1906
Misattributed
Variant: A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty.

United Nations General Assembly - Promotion of a democratic and equitable international order http://www.ohchr.org/Documents/Issues/IntOrder/A-68-284_en.pdf.
2013

“As a nation we must prevent hunger and cold to those of our people who are in honest difficulties.”
The Hoover Policies (1937)

Source: 1860s, Evidence as to Man's Place in Nature (1863), Ch.2, p. 71

Quote (June 1902), as cited in Artists on Art, from the 14th – 20th centuries, ed. Robert Goldwater and Marco Treves; Pantheon Books, 1972, London, p. 442
1895 - 1902

In page=89
Remembering Our Leaders: Mahadeo Govind Ranade by Pravina Bhim Sain

Author's Preface
On the Study and Difficulties of Mathematics (1831)

Source: Mathematical Lectures (1734), p. 31: Prefatory Oration

“The price of corn will naturally rise with the difficulty of producing the last portions of it,…”
Source: The Principles of Political Economy and Taxation (1821) (Third Edition), Chapter XXXII, Malthus on Rent, p. 276

Systems Movement: Autobiographical Retrospectives (2004)

Source: On Building Systems That Will Fail (1991), p. 80

Letter to George Washington (November 1779)

La principale difficulté pour vulgariser la physique quantique, c'est qu'on ne sait pas très bien comment en fabriquer des images dans notre monde. C'est en ce sens qu'elle est vraiment contre-intuitive.
Interview http://www.canalacademie.com/Alain-Aspect.html on the occasion of the CNRS Gold Medal Award Ceremony in December 2005.

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 146.

For My Legionaries: The Iron Guard (1936), Politics

Concurring, Cooper v. Aaron, 358 U.S. 1 (1958).
Judicial opinions