“We must welcome the future, remembering that soon it will be the past; and we must respect the past, remembering that it was once all that was humanly possible.”

Pt. III : The Positive Aspect of Ambiguity http://www.marxists.org/reference/subject/ethics/de-beauvoir/ambiguity/ch03.htm#s2, Ch. 1 : The Aesthetic Attitude
The Ethics of Ambiguity (1947)
Context: We must not confuse the present with the past. With regard to the past, no further action is possible. There have been war, plague, scandal, and treason, and there is no way of our preventing their having taken place; the executioner became an executioner and the victim underwent his fate as a victim without us; all that we can do is to reveal it, to integrate it into the human heritage, to raise it to the dignity of the aesthetic existence which bears within itself its finality; but first this history had to occur: it occurred as scandal, revolt, crime, or sacrifice, and we were able to try to save it only because it first offered us a form. Today must also exist before being confirmed in its existence: its destination in such a way that everything about it already seemed justified and that there was no more of it to reject, then there would also be nothing to say about it, for no form would take shape in it; it is revealed only through rejection, desire, hate and love. In order for the artist to have a world to express he must first be situated in this world, oppressed or oppressing, resigned or rebellious, a man among men. But at the heart of his existence he finds the exigency which is common to all men; he must first will freedom within himself and universally; he must try to conquer it: in the light of this project situations are graded and reasons for acting are made manifest.

2015, Bloody Sunday Speech (March 2015)
Context: We respect the past, but we don’t pine for the past. We don’t fear the future; we grab for it. America is not some fragile thing. We are large, in the words of Whitman, containing multitudes. We are boisterous and diverse and full of energy, perpetually young in spirit. That’s why someone like John Lewis at the ripe old age of 25 could lead a mighty march. And that’s what the young people here today and listening all across the country must take away from this day. You are America. Unconstrained by habit and convention. Unencumbered by what is, because you’re ready to seize what ought to be. For everywhere in this country, there are first steps to be taken, there’s new ground to cover, there are more bridges to be crossed. And it is you, the young and fearless at heart, the most diverse and educated generation in our history, who the nation is waiting to follow.

Source: Social Amnesia: A Critique of Conformist Psychology from Adler to Laing (1975), p. 5

Speech at Zurich University (September 19, 1946) ( partial text http://www.churchill-society-london.org.uk/astonish.html) ( http://www.peshawar.ch/varia/winston.htm).
Post-war years (1945–1955)

Sydney, (22 January 2017)[citation needed].

Variant: After all, Bao Bomu says, what is the past but what we choose to remember?
Source: The Bonesetter's Daughter

Source: Elantris

Source: Tomorrow Is Now (1963), p. xv
Context: We face the future fortified with the lessons we have learned from the past. It is today that we must create the world of the future. Spinoza, I think, pointed out that we ourselves can make experience valuable when, by imagination and reason, we turn it into foresight.

Speech at Founding Rally of the Organization of Afro-American Unity (28 June 1964), as quoted in By Any Means Necessary (1970)
By Any Means Necessary (1970)