But the problem is that bad writers tend to have the self-confidence, while the good ones tend to have self-doubt.

Charles Bukowski (1920-1994) German-American author, poet

“Charles Bukowski,” interview by Alden Mills, Arete (Jul/Aug 1989)

This is almost always misquoted in a much broader paraphrase, e.g., "The problem with the world is that the intelligent people are full of doubts, while the stupid ones are full of confidence," perhaps to echo Russell and Yeats.

More examination of this quotation: The Best Lack All Conviction While the Worst Are Full of Passionate Intensity – Quote Investigator.