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.
I spend all night scared of tomorrow, only reason I aint killin, suns up, when it gets dark im back to the park, cut through the back im after the heart, creepin through your hallway with a knife, had it all planned, therapist callin me all day, I know you love it when I eat meat, I love it when the brains hit the concrete!