“I have never started a fight in my life but I have finished a few… sometimes I shudder when I think about what I have done.”

Reported in his Tennessean's obituary; quoted in "John Seigenthaler dies at 86" http://www.poynter.org/2014/john-seigenthaler-dies-at-86/258597/ by Andrew Beaujon, poynter.org (11 July 2014)

Address at Bennington College (30 October 1984) as published in "Reflections of a Writer: Long Work, Short Life" in The New York Times (20 March 1988)
Context: I have written almost all my life. My writing has drawn, out of a reluctant soul, a measure of astonishment at the nature of life. And the more I wrote well, the better I felt I had to write.
In writing I had to say what had happened to me, yet present it as though it had been magically revealed. I began to write seriously when I had taught myself the discipline necessary to achieve what I wanted. When I touched that time, my words announced themselves to me. I have given my life to writing without regret, except when I consider what in my work I might have done better. I wanted my writing to be as good as it must be, and on the whole I think it is. I would write a book, or a short story, at least three times — once to understand it, the second time to improve the prose, and a third to compel it to say what it still must say.
Somewhere I put it this way: first drafts are for learning what one's fiction wants him to say. Revision works with that knowledge to enlarge and enhance an idea, to re-form it. Revision is one of the exquisite pleasures of writing: The men and things of today are wont to lie fairer and truer in tomorrow's meadow, Henry Thoreau said.
I don't regret the years I put into my work. Perhaps I regret the fact that I was not two men, one who could live a full life apart from writing; and one who lived in art, exploring all he had to experience and know how to make his work right; yet not regretting that he had put his life into the art of perfecting the work.

Comments on his final election defeat (11 August 1835) Ch. 2; in Dr. Swan's Prescriptions for Job-Itis (2003) by Dennis Swanberg and Criswell Freeman, p. 45, part of this seems to have become paraphrased as "Let your tongue speak what your heart thinks." No earlier publication of this version has been located.
Col. Crockett's Exploits and Adventures in Texas (1836)

Happy birthday, Tall Man! ‘Phantasm’ turns 30 https://herocomplex.latimes.com/uncategorized/phantasm/ (October 16, 2009)

Life in the Industry: A Musician's Diary

As quoted in Journey Through Womanhood: Meditations from Our Collective Soul (2002) by Tian Dayton

Quoted in Albert Jay Nock's Memoirs of a Superfluous Man (1943), p. 54.
Attributed

"An Interview with Gwendolyn Brooks", Contemporary Literature 11:1 (Winter 1970)

Source: Twitter https://twitter.com/repjohnlewis/status/1207420638748725250, (30 December 2019)