[Edit: Beaten to it by goblgobl....]
My mind always dredges that quote up when I start to question my abilities. (Which I've been doing a bit of lately, coding for a nearly 15 year old Windows NT system. Relearning things I haven't seen since the '90s....)
Certainly we all have moments where we question our abilities. Small crises of faith, where our self confidence withers away. It's just part of life. Part of the checks and balances of our brain.