"Please pray specially for me that I may not spoil His work and that Our Lord may show Himself -- for there is such terrible darkness within me, as if everything was dead," she wrote in 1953. "It has been like this more or less from the time I started 'the work.'"

That's Mother Teresa. Apparently even she doubted and was depressed, apparently for much of her career. It didn't stop her from raising huge amounts of money for the church and eventually being beatified.

www.google.co.uk/search?q...resa+doubt