My cat is only 3 and I worry about her getting older and what I will feel when she dies. I would like to retire outside the US in 15 years, but I won't leave until after she passes, which to me is kind of a big decision to make based on a pet, but if it means I'm here another 20 years, then so be it (assuming I live that long!).
I try to make myself realize that my loved ones will die, too, trying to desensitize myself in my mind so it won't be so bad when it happens. It helped a little in the case of my grandfather. I know exactly what you mean about wishing something could last forever. I make what I think of as mental snapshots of those moments and keep them in my head to review whenever I miss them. So if doing that is crazy, at least you're not crazy all by yourself.
I've read several accounts of older kitties being mouthy and one friend's vet suggested, after ruling everything else out, that the cat was suffering some senile dementia, and that made sense to me. I don't know if it helps. For some reason, once it made sense to me, I was not so upset on my friend's behalf or so tortured by wondering what her cat wanted with all that howling.
I'm sorry for your losses. It's hard to let go sometimes so be gentle with yourself about it.