“I regret how much I believed in the future.”

Thoughts on Man's Purpose in Life (1974), Exchange with Admiral Rickover (1982)
Context: I do not have regrets. I believe I helped preserve the peace for this country. Why should I regret that? What I accomplished was approved by Congress — which represents our people. All of you live in safety from domestic enemies because of security from the police. Likewise, you live in safety from foreign enemies because our military keeps them from attacking us. Nuclear technology was already under development in other countries. My assigned responsibility was to develop our nuclear navy. I managed to accomplish this.

"Agnostic".
Rhymes for the Irreverent (1965)
Context: No matter how much I probe and prod,
I cannot quite believe in God;
But oh, I hope to God that He
Unswervingly believes in me.

Union Bank v. Munster (1887), L. R. 37 CD. 54.

Source: Extremely Loud and Incredibly Close

His last letter, to Myron Benton (31 March 1862) http://www.walden.org/Institute/thoreau/writings/correspondence/1862_03_21_Benton.htm
Context: You ask particularly after my health. I suppose that I have not many months to live; but, of course, I know nothing about it. I may add that I am enjoying existence as much as ever, and regret nothing.

Until When We Are Ghosts (2006), My Life Changed