I think it is worth remembering that people who complain are really sad and unhappy.
I suppose you could blame them for not doing more to help themselves, or for feeling helpless about their situation, but I've always thought people reach a point where they probably think that they've tried every pain medication but their back still hurts, and they've tried every trick to overcome insomnia and none of them really worked either, and they know they should join some groups to make friends but it's just too scary.
Fundamentally, she's sad and the things that are hurting her - whether we get it or not - are all consuming and at the forefront of her mind at all times. She knows no one cares about these things, not really, but desperately wants someone to genuinely hear her and realise what she's coping with, just like her husband used to, or her good friends before they moved away.
I often think people living alone and unhappy could do with a good therapist for a weekly moan to people paid to care.
Unfortunately you can't simply list all of the things she has that others don't have, all the things she should be grateful for, because sadness doesn't work like that.
And you shouldn't wonder why she can turn on the charm for people outside the family, because we are all at our worst with the people we trust aren't, when we can be ourselves without the public mask - it's why children are sometimes angels at school but awful at home.
Just grin and bear it. Only give as much of yourself as you can. Remember that she loves you and doesn't want to be a burden, and will be gone soon enough. When you are her age you might understand her better.