I’m so sorry about everything that is going on for you, nannytothequeen, it sounds like an almost intolerable burden.
And I’m furious for you about your crap friend. It’s true that she may be finding it all too much to support you, but there are better ways to do self-preservation while staying supportive.
Can you bear to address it directly with her? Maybe acknowledge that you’ve been leaning a lot on her, apologise for it being too much and lay out a plan to continue the friendship in a more measured way?
This is not what you SHOULD do,because she has behaved badly, but it often helps to acknowledge what led to her bad behaviour.
This is only if you feel able to forgive her failure and would still value her friendship. It would be a LOT to ask of yourself, and goodness knows you are already drawing on all your reserves.
What you should also do, though, is turn to your other friends. You don’t have to tell each one everything, but do lean on each of them as much as you feel you can.