Namsnanny I am very sorry to hear how you are suffering. You certainly are not alone, there are many grans and grandpas who suffer badly because of Adult Children. I am one too.
I have to say some things you won't want to hear, I am sorry. You said; I've been doing EVERYHING Nearly has required of me. The first thing therefore is to STOP pandering to this selfish and untrustworthy woman. You are going to need to step back. I know you love her. I love my children. But I still can't change the one who deliberately hurts me and who appears to have a personality problem. Ask yourself, if you were her age and met her, say at a group for Mums and children, would you like her, assuming that somehow you also knew how she treated her mother? Would you be friends with someone who behaves like this? You are being manipulated. I cannot say what her problem is but I imagine if you googled some of her most salient characteristics and behaviours you would be shocked about what would come up. You are going to need to show her you do not need her. It will mean you see the DGC less often. On that point, as some others have said, you are expecting to see them far more often than many of us see ours. Admittedly I live a long way away, but I see my GC about once or twice a year. I've had to come to terms with that. I have to build up my own life independently of my children.
You have a difficult AC who will continue to torture you the more you feed her with your interactions. If you give her the silent treatment it will be the shock of her life for her, she really will not believe that you can live without her. Then she might get her act together. But don't expect much. People with her nature seldom change. They may look as if they have become kind and loving but do be carful, they can put on a very convincing act to get what they want.
I am so sorry to be so blunt, but I do think your best way forward is to build up your life independently of this AC. She will only give you more distress if you don't. If you find you don't really need her she may start to treat you with respect.
Wishing you strength and happiness. Please start planning nice things to do for yourself, little projects you like to do, seeing your friends, being happy with your life independently of the offspring. They'll come to you when they need you, have no fear. With love, L 