I've sat in my local support group and watched people struggle with accepting that their family member has this disease. I've seen it over and over. I've seen it here on TP as well. It's an awful disease and we don't wish it on anyone, least of all someone for whom we care.
Last year my FIL was diagnosed with terminal cancer, an aggressive type, which had metastasized. He was going to live, at best, six months past diagnosis.
My poor MIL simply could not wrap her head around this. As he got sicker and sicker, she somehow didn't see his frailty and illness. She was so unable to accept this, that she wouldn't even use the word "cancer." He just wasn't feeling well. She refused to go to any of the doctor's appointments or to schedule them. She refused to go to his chemotherapy sessions. She refused to discuss it with any of her adult children.
It was terribly sad, but it was such a shock that she just couldn't process it, and then the fluster of activity around diagnosis and treatment made it even harder. It took her almost four months to come to terms with his diagnosis.
So I think some people just need more time, no blame or shame, it's just how we are. But I can see that this is frustrating, especially if things could be better or different, with more acceptance and understanding. (I know I felt that way about my MIL.)
I do think some kind of support group or lecture or coffee meeting for carers, might give her a sense she's not alone (and so many of us carers feel that) and maybe some perspective? But it's hard, and I'm sorry.