Stumbled upon a great website recently: Finding Mastery with Michael Gervais.
He has a great episode with Mark Divine, who has wide background in physical and mental training, including designing training programs for Navy Seals.
One quote from the episode really struck a chord with me:
“Mastery is a progressive process of integration refining the physical, mental, emotional, intuitional, spiritual until they emerge as one.”
and check out one of his articles from his "Unbeatable Mind" website:
"What is Mental Toughness and Why is it Important?"
(I love "Embrace the Suck"...)