I suppose "memento mori" can be argued in the worst sense as leading to nihilism. However, it'd be impossible to say anything of substance if we were constantly trying to avoid any worst-case interpretations. Most reasonable people can surely understand there's a difference between the colloquial usage of death versus suicide.
I think it’s not possible to understand unless you know the feeling. You’re right that most people don’t. But many do: Robin Williams, David Foster Wallace, Aaron Swartz.
I don’t claim to speak for them, but if any of them used it as an escape — which seems somewhat likely - then they’re examples that the logic can be turned against someone strong, using their strength against them by encouraging them to accept the inevitability with composure.
You’ll have to take my word that there are many more of us than those famous examples, though.
About 3.8% of the world population, or about 300 million people are affected by depression (WHO 2021). It seems safe to assume that (1) the true figure is under-reported and (2) those affected are connected to others who can also deeply relate in some way.
Personally, I know all too what what that feeling is like. The solution for me was finding people who could speak truth with love, and just time. Knowing that we're all in the same boat gives me more -- not less -- reason to make the ride count.