“Sometimes the most important thing in a whole day is the rest we take between two deep breaths.”

Bowdoin Academic Spotlight interview (2011)
Context: You have to take advantage of the opportunities that life gives you, particularly the moments in time when you have time, when you're between jobs or you retire. Get out and go. I think most of us are way too intense. We need to take a deep breath and do things that maybe don't fit the normal picture of what we're supposed to do at that stage of life. In some ways, this book is one big argument for just plain loosening up.

Source: Myatt, David. Myngath - Some Recollections of the Wyrdful Life of David Myatt, CreateSpace, 2013, ISBN 978-1484110744

Alleged to have been made in a September 13, 2001 press conference. This wording has not been confirmed.
Attributed, Misquotations

Variant: If neurotic is wanting two mutually exclusive things at one and the same time, then I'm neurotic as hell. I'll be flying back and forth between one mutually exclusive thing and another for the rest of my days.
Source: The Bell Jar (1963), Ch. 8

Source: https://quotepark.com/quotes/2101720-kimsa-sok-life-is-a-journey-to-be-experienced-not-a-problem/

This appears on the opening placard of the film The Equalizer, attributing it to Twain, but there is no evidence that Twain wrote it. A precursor is found in Taylor Hartman's self-help book The Character Code (first published 1991), where it is not attributed to Twain: "The three most significant days in your life are: 1. The day you were born. 2. The day you find out why you were born. 3. The day you discover how to contribute the gift you were born to give" ( Google Books link https://books.google.com/books?id=gIKCxWxNmeMC&pg=PA147&dq=%22day+you+find+out+why%22&hl=en&sa=X&ved=0ahUKEwijrJzc84vLAhUJzGMKHajvADEQ6AEIIjAB#v=onepage&q=%22day%20you%20find%20out%20why%22&f=false)
Disputed

Source: Natural Right and History (1953), p. 36

Source: What I Talk About When I Talk About Running