“Even the most confident maintain an edge of doubt and fear; for they are the engines that remind us that confidence is a necessity for success.”
1985
Related quotes

“Kill your doubt with the coldest of weapons: Confidence.”
Lyrics, Light Grenades (2006)

“Troops are not to be led to battle unless confident of success.”
Numquam ad certamen publicum produxeris militem, nisi cum eum uideris sperare uictoriam.
De Re Militari (also Epitoma Rei Militaris), Book III, "Dispositions for Action"
Context: An army is strengthened by labor and enervated by idleness. Troops are not to be led to battle unless confident of success. (General Maxims)

“Self-confidence is an important ingredient that makes for a successful physicist.”
[Victor Weisskopf, Growing up with quantum field theory: The development of quantum electrodynamics. In: The birth of particle physics (ed. L.M. Brown and L. Hoddeson), Cambridge University Press, 1983, 75]
“Turn your confidence and your fears alike into prayer.”
Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 600.

“Fear only has one enemy and that is a confident persona.”
WWE Hall of Fame induction (2019)

“All you need in this life is ignorance and confidence, and then Success is sure.”
Mark Twain's Notebook, 1887
Letter to Cordelia Welsh Foote (Cincinnati), 2 December 1887. Letter reprinted http://www.twainquotes.com/Success.html in Benjamin De Casseres's When Huck Finn Went Highbrow https://www.worldcat.org/title/when-huck-finn-went-highbrow/oclc/2514292 (1934)