Well... you know... I'm pushing 70 years old!

And I never expected (or wanted) to live this long. When I hear or read something about how stress shortens your life span, I just have to chuckle because that certainly hasn't been true in my case. I've lived with an ocean liner's quantity of stress

my whole life & I'm still here... & I'm even physically healthier than probably the majority of people my age. (Go figure...) Life has a way of fooling us.
I'm not a mental health professional.

So anything I write here is just my personal opinion. But, from my perspective, what you are struggling with is just what depression does to a person.

(You probably didn't need me to tell you that.)

And, likewise, you probably don't need me to tell you that the standard treatments for depression are therapy of one variety or another (individual, CBT, DBT, etc.) & / or antidepressant medications. Oh... & then there are such things as ECT & TMS as well as probably a number of other things that wouldn't typically occur to me.
The thing is... your life... however long it lasts... will be whatever you make it... for better or worse. Yes, perhaps you're correct & you won't live that much longer. Or perhaps you'll be like me & you'll just keep hangin' on for decades!

There's no way to know for sure. But every human being in the world is in the same boat. We could live on for decades; or we could unexpectedly collapse in a matter of seconds. Consequently all any of us can do is to go on day-after-day as if we're here for the long haul & see how things work out. And we can either do that in an optimistic, constructive way, or we can just hide out & wait for it all to be over. We get to choose.
Of course the question is, how do you carry on in an optimistic, constructive manner when you're drowning in depression. Sadly, I don't have any really great answers to that question. As I wrote above, the standard answer is therapy & / or medication. Beyond that, I think you have to find something in your life that you love well enough that it makes it possible for you to overcome the depression you're mired in (possibly with the help of therapies & medications.)
The thing is, at least from my perspective, there aren't any secret treatments out there... no miracle cures for what you're experiencing; something that if you could just find out about it would suddenly make everything better. The options are pretty generally known. In the end, it simply comes down to a matter of you deciding how you will conduct your life with whatever amount of time you have. My best wishes to you...