I agree with Rabbits. I have a similar mother. Like you, I’ve tried to get her to own her problems and get therapy or support, as well as trying to help her find social activities etc, but she simply won’t do it and prefers to sit at home all day bring increasingly bitter and nasty about the world and, especially, about her family (who in her mind have all failed her by not somehow making all her problems vanish through some unspecified means).
There’s lots more to it than that but that’s the basic outline.
You have to detach and really acknowledge that she’s an adult and has chosen to live this way, if by no other means than refusing therapy or anything else that might change her mindset. And focus on you. Decide what level of contact you want with her and stick to it. Perhaps have some counselling to help grieve, but in any case I find it helpful to acknowledge that grief comes in waves and there are better times. I’ll never get over having a mother like this but I find ways to live with it and find my own happiness where I can.
She’s never been a close or nurturing mother, so at least I don’t have the pain of having lost that (though I do wonder if your memories have something of a rose tint about them? My sister also says she misses nice kind Mum but I have no memories of that person ever existing, and I’m the older one).
There is a social pressure to pretend everyone’s got a lovely close mum, but in reality a lot of us live without ever having known that. I just try to be the kind and nurturing mother to myself that I never had, for instance by challenging negative self-talk and trying to look after myself well. And having good female friends who are kind to me as I am to them.
I can remember my grandmother being like my mother is now, when she got old. My biggest fear is that I will go the same way. I’m trying to do a lot of work on myself, coping skills, resilience, social connection, mindfulness.... to give myself the best chance of avoiding it.