I strongly believe that people are held back because they dwell too much on their shortcomings rather than throw their hats in the ring and show up.
We run simulations in our heads about why things will go wrong or how we're not ready for said challenge and use that to justify not accepting challenges. More often than not, these are challenges we would have proffered solutions to if we gave a good attempt.
I had a personal remind of this yesterday when I was asked for a favor, my mind was running simulations around how difficult the problem would be. Turns out I solved the problem in three minutes and was shocked that i knew that much.