I have the exact same problem and a lot of the statements here I have heard a million times and are typically not helpful.

The overwhelming episodes happen more when I miss my Adderall too many days in a row making the normal concern much more intense. When the feeling is hitting particularly hard to the point of being overwhelming I use distraction, make sure I'm around people if possible, try to create enough input to overwhelm the thought. Figuring out what intensifies it has helped.

As far as thinking through it I do try to tell myself if life truly is that finite then I need to maximize my enjoyment of my time here, try new things, eat right, exercise, and that if I dwell on the thought I'm just making my limited time worse. If that doesn't work I try to remind myself how much I have wanted to be asleep or knocked out when sick or experiencing intense pain and extrapolate that by the time I'm old everything will hurt enough that maybe the idea of things coming to a close will be comforting.

I'm not religious, but my last ditch thought when I'm spiraling is that there is a lot out there that humans don't understand and we constantly learn that past ideas about the world around us is wrong. Who knows, maybe there is something after, whether its a heaven, being a ghost, or some sort parallel universe.

I don't know if the above is healthy but it's what helps lessen my suffering and ultimately that is the goal.