Be cautious with self-deprecation.
Do not speak badly of yourself, for the warrior that is inside you hears your words and is lessened by them.David Gemmell, Quest for Lost Heroes, 1990
It seems this quote has several attributions. But the earliest reference I could find was from David Gemmell, Quest for Lost Heroes. That said, if somebody could cite a Samurai Proverb source earlier than 1990, I’m all ears.