I feel exactly the same way about everything. I went to college for a while, only to drop out and get stuck with student loan debt for nothing. I've worked at jobs that I hated with the goal of making enough money to take care of myself and be happy, but that hasn't worked out so far (but I'm still trying, because I can't CTB yet). I also am terrified about the idea of growing old, just like you and Miguel.
Even if I was able to obtain everything I have ever wanted and life magically became better, I know that it wouldn't last. The thing that seems the most pointless of all would be to work my life away until I am old enough to retire, which a lot of people do. If I do that, should I try to enjoy it before I die of cancer or develop dementia? What if I die of an illness BEFORE I am old enough to retire? Then trying to work for retirement would be a total waste of time. Old age would ruin everything anyway, even if I did get to enjoy it for a while, so it makes sense to enjoy life as much as possible while we are still young and end it if doing so is impossible.
I feel like dying at the age of 30 would be perfect for me. That gives me 3 years, but sometimes it feels a lot longer than that. I suppose if I was content and happy then, maybe I would stick around longer than that, but 35 is probably where I will have to draw the line. If the family members that I am afraid of hurting are still alive, I guess I will have to find a way to write an apology note for them. I just don't know what I would say if I did that.