I can relate.
Most of the time, I am somewhat optimistic about my future, but the older I get, the less that is true. I often feel that I'm not friends with Life anymore. It seems so incredibly unfair with only hurt and despair to offer. Joy seems fleeting so often. No, like you, I won't ever do anything to cause early demise for myself - after all, that would hurt many people close to me... but I do understand - and you know the higher your IQ, the more prone we are to existential depression...

It's one of the reasons why I write fiction, I guess. I can create characters with a greater future than what I feel I have for myself.

It can be hard at times. A lot of the time...
Hang in there - and experience what you can. THAT is what we are here to do.