Memento Mori – remember you must die. The oldest trick in the Stoic toolbox, and still the one that cuts deepest when you actually let it land.
It’s not meant to be morbid. The point isn’t to sit around thinking about corpses, it’s to use the fact of your mortality as a filter for what actually matters. You could leave life right now – not today necessarily, but at some point, and probably sooner than you think. Knowing that changes the weight of things. Most of the stuff you’re worrying about right now will not matter in a year, let alone at the end. That’s not depressing, it’s liberating.
I find it useful for two things: keeping my head about the good bits so I don’t get arrogant, and letting go of the bad bits without spending weeks fuming about them. Both are harder than they sound. We’re not naturally disposed to treat our successes as temporary or our inconveniences as trivial, but the exercise is there if you use it.
It pairs nicely with amor fati – remember you will die, and love whatever happens to you while you’re here. One without the other is just anxiety or just fatalism.