Confidence’s Etymology, as I see it is:
Con: Together, Together with, In combination with
So, having confidence implies that you have an ample faith to believe in yourself, believing that you have the power to accomplish things. To face challenges boldly.
Faith comes from the inside isn’t it? Everybody has it. Even Nihilists does (They have faith in their unbelieving).
Whenever you feel like you can’t do it, just remember your faith. Whatever form it might be. It’s an everflowing source of energy.