On struggle.
Simply put, the vast majority of us are not used to struggle, not in a real and tangible way. Sure you all have your woes about finances, relationships, getting over a bad habit or condition. But that's most often something that could be solved with a month or two of not being a gigantic bitch or otherwise just trying real hard for once.
What I mean is struggle on par with what's been the historical norm for 10,000 plus years. Hunger, violence, trials of will and resolve, loss and horror, disease, seeking your next shelter, and so on.
THAT, is something almost everyone is not only not accustomed to, but actively averse to experiencing altogether.
You MUST kill this idea in your mind. Seek this struggle out instead.
Seek out hardship, embrace struggle, and crave being uncomfortable. Nearly every other sad sack of biomass out there in society today goes about their days and lives not wanting to degrade their perfect little existence in the slightest little way, with if and when it does causing them to break as easily as a toothpick.
There are innumerable ways to harden yourself; gym, fasting, spontaneous hiking with only what you've got packed, hunting, the goruck challenges, one Shepard, combat survival courses, or just making yourself be OK with being out of comfort. Like intentionally keeping your place at an uncomfortable temp, sleep without a pillow, intermittent fast and see what effects it has, etc.
To close, none of this has to be an all encompassing thing from now until you die. But you've got to understand that you're weaker for not having struggle, we all are, and we make ourselves just that little bit better tomorrow by suffering a tiny bit today.
So go do a 4 mile run and take a fucking cold shower.
RB