Work is the keystone of a perfect life. Work and trust in God. We are what we repeatedly do, excellence then is not an act, but a habit. The man who does not shrink from self-crucifixion can never fail to accomplish the object upon which his heart is set. This is true of earthly as of heavenly things.
Mental toughness is many things and rather difficult to explain.
Its qualities are sacrifice and self-denial. Also, most importantly, it is combined with a perfectly disciplined will that refuses to give in. It's a state of mind-you could call it character in action. There are no short cuts to any place worth going. Never excuse yourself.