How to stop overthinking if life is short ?

Don't sweat the small stuff. - Keep it light. - So what?

you can worry about this but then you will miss out on today.  Live every day as if it is your last, no one is guaranteed a tomorrow.  I lived 35 years with my husband when he passed it was as if we had just met.  In this sense life is very short indeed.

the easiest way to stop overthinking is to have a busy productive life 

if you have things to do, your mind is less likely to focus on all the random thoughts in your head 

Mayflies live only for a day or so. Yet they think that's eternity...

Consider the idea...

Time is finite; you live for only a short-ish time. So, you consider this a lack, and bemoan the lack. But people can achieve a lot in that short span. Not everything, but enough to make a difference.

So where are YOU spending your time to make a difference, and what is that difference?

If you know you make a difference, then time or the lack of it is not important.

Keep on keeping on!

And, as Dylan Thomas put it: rage against the dying of the light! (but never be afraid of life ending)