“Every chance is an opportunity!”

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.

Book IV, Ch. 31: The Promise of America
You Can't Go Home Again (1940)

Marianne Means (September 26, 1986) "I Just Flew In From The White House - And, Boy, Are My Arms Tired", Seattle Post-Intelligencer, p. A10.

Maiden speech on 24 May 2005

Speech in the House of Commons, June 4, 1940; passage praising the airmen of the Royal Air Force and their efforts during the evacuation of Dunkirk. This is a close paraphrase of Tennyson:
When every morning brought a noble chance,
And every chance brought out a noble knight.
Alfred Tennyson, "Morte d'Arthur" http://home.att.net/~TennysonPoetry/mort.htm, stanza 23 (1842), and the expanded "The Passing of Arthur", stanza 36 in Idylls of the King (1856–1885)
The Second World War (1939–1945)

Source: Morte D'Arthur (1842), Lines 230-231

Out of Control: The New Biology of Machines, Social Systems and the Economic World (1995), New Rules for the New Economy: 10 Radical Strategies for a Connected World (1999)

1977