Challenge Yourself
I read several books on mental toughness and to be honest there is no formula for this because you are a creature of your environment and only you can develop this based on the series of circumstances that you dealing with. Everyone is dealt different cards in life and that's what makes us unique. I guess figuring out what makes you a fighter is really based on the demons you carry around inside you. So, I'm not trying to be cliche. Just find your place in life and figure out what you want then just make it work.