“The true optimist not only expects the best to happen, but goes to work to make the best happen. The true optimist not only looks upon the bright side, but trains every force that is in him to produce more and more brightness in his life…”

Source: Your Forces and How to Use Them (1912), Chapter 10, p. 155

This is derived from a statement of James Branch Cabell, in The Silver Stallion (1926) : The optimist proclaims that we live in the best of all possible worlds; and the pessimist fears this is true.
Misattributed
Variant: The optimist thinks this is the best of all possible worlds. The pessimist fears it is true.

This is actually James Branch Cabell from The Silver Stallion (1926)
Misattributed

Coth, in Book Four : Coth at Porutsa, Ch. XXVI : The Realist in Defeat
Source: The Silver Stallion (1926)
Context: Yet creeds mean very little... The optimist proclaims that we live in the best of all possible worlds; and the pessimist fears this is true. So I elect for neither label.

A Clarification (March 24, 2016)

Source: Song written for the film, Monty Python’s Life of Brian (1979)

"Coon Tree," The New Yorker (14 June 1956), The Points of My Compass: Letters from the East, the West, the North, the South (1962); reprinted in Essays of E.B. White (1977)

On his hopes for mankind in “In the Author’s Universe: Interview with Sci-Fi Author Cixin Liu” https://vocal.media/futurism/in-the-authors-universe-interview-with-sci-fi-author-cixin-liu in Vocal (2016)