Mental toughness is the measure of how much you're willing to put yourself through to achieve your desired goals. Found in droves in athletes, especially in contact sports like football and mma thanks to the very nature of those activities. But running into another peak athlete at the highest speeds that you can build or standing toe-to-toe in a cage with someone trying to knock your head off aren't the only ways to build up your mental toughness.
There are exercises, physical and mental, to put yourself through that can build up your mental toughness. Like the fighter Chael Sonnen said, after putting yourself through enough misery you start to expect a victory at the end of the road.
One step you can take is to device a workout specifically designed to be a miserable experience. And you must force yourself through it. A good trainer can make one of these for anyone, but if you're doing this by yourself it can be tough, and you'll need to really stick it out to finish a few of these sample workouts.
Farmer's Carries. They're a workout that consist of the heaviest dumbbells, or kettlebells, or even just free weights, chains, or cinder blocks that you can carry and going for a long walk. And when I say long, I mean up to a mile to two and a half. If you want to rapidly build both your grip strength and your ability to handle physical duress this is a sure-fire way to do it.
A quicker one would be a workout called the 3-2-1 back squats, a simple and brutal workout meant to be done and finished in nine minutes. It consists of the following:
3 minutes: As many reps as possible without racking the bar.
2 minutes: Rest
2 minutes: As many reps as possible without racking the bar.
1 minute: Rest
1 minute: As many reps as possible without racking the bar.
Now, the weight doesn't have to be brutally heavy, thirty percent of your max squat will be fine. And even if it feels light at first, you'll quickly realize how little you'll be wanting to do this workout. As a quick aside, make sure to only go to the limits of good form. That goes for any of these exercises.
And finally, a workout for the chest. Push ups and plate-pushes. This workout is a great way to burn fat, increase cardio, and remind yourself that you're willing to work through the pain of life (excuse my dramatic flair). The work consists of sets of 20 push ups, followed by a 20-yard plate push...which is honestly just pushing a 45-pound plate across the floor. Make sure to look up a form video as to avoid any back injuries. For rest, five deep breaths between each. Continue the workout until push ups fall below 10 reps or you can't move the 45-pound plate anymore.
But remember, the true key to developing mental toughness is consistency. If you only do these once in a blue moon, sure they'll be a good workout but you lose the real purpose of the workout, and that's to, quite bluntly, harden yourself to hard work and physical pain.