Self-belief does not necessarily ensure success, but self-disbelief assuredly spawns failure.
Bandura, Albert (1997). Self-efficacy: The exercise of control. New York: W. H. Freeman. ISBN 9780716728504. (p. 77)
If self-efficacy is lacking, people tend to behave ineffectually, even though they know what to do.Albert Bandura
Persons who have a strong sense of efficacy deploy their attention and effort to the demands of the situation and are spurred by obstacles to greater effort.Albert Bandura