Did Mark Twain really say it?
Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do.
Really: H. Jackson Brown Jr., 1990
Twain never wrote it. It first appears in H. Jackson Brown Jr.'s 1990 book P.S. I Love You, as advice from his mother.
How to post it: Credit H. Jackson Brown Jr.'s mother, in P.S. I Love You.