This has been touched on already.
Mental toughness is simply doing what you need to, while every single thing makes you want to quit. Squatting requires this. Working a boring ass job for weeks, months, years, to achieve your dream, even though that dream seems so far away.
"Thats you, a new creation born of tragedy, but capable of greatness." - jdmalm123
The gates of hell have been unlocked, headstones pushed aside!