Marcus Aurelius, answered from the texts and cited to the page.
Death is an operation of nature — nothing more, and nothing less.1 A child fears it because he has not yet looked at it directly, has not yet resolved by reflection what the imagination makes monstrous into its actual parts. Do that work, and what remains is simply: a thing happens, as digestion happens, as sleep happens, as the dispersal of smoke into air happens.
The practice you are naming — memento mori, remember that you will die — is exactly this: the daily, deliberate turning of the mind toward what the mind instinctively avoids. Not to cultivate misery. To see clearly. Because what clouds judgment is not death itself but the false weight we assign to the things death will take: the reputation, the pleasure, the body, the applause of people whose own names will be forgotten in a generation.2
Strip those things of their borrowed glamour — see the purple robe as dyed wool, the famous man's legacy as vapor — and the terror of losing them dissolves with the glamour. The daemon within, the ruling faculty, is what deserves your attention.3 Not the noise outside. When you remember you will die, you are not practicing despair; you are practicing the redirection of attention from what is perishable and not yours to what is actually yours — the quality of your judgment, the disposition of your soul, the reverence you bring to each hour.
Death levels the good man and the bad equally; it is indifferent to virtue and to vice.4 That indifference is not a cruelty. It is the proof that death is not an evil, because what happens equally to the virtuous and the vicious cannot be counted among the things that make us better or worse. Remember you will die, then. Not once, as a dramatic gesture. Every morning, as a practice. The remembrance is the medicine; it works only by repetition.
if a man looks at it in itself, and by the abstractive power of reflection resolves into their parts all the things which present themselves to the imagination in it, he will then consider it to be nothing else than an operation of nature; and if any one is afraid of an operation of nature, he is a child.Meditations
How quickly all things disappear,--in the universe the bodies themselves, but in time the remembrance of them. What is the nature of all sensible things, and particularly those which attract with the bait of pleasure or terrify by pain, or are noised abroad by vapory fame; how worthless, and contemptible, and sordid, and perishable, and dead they are.Meditations
it is sufficient to attend to the daemon within him, and to reverence it sincerely.Meditations
death certainly, and life, honor and dishonor, pain and pleasure,--all these things equally happen to good men and bad, being things which make us neither better nor worse.Meditations