Ziggy62, one of my own daughters has, I think, undiagnosed dyspraxia. She's suffered from depression over the years and can be quite volatile. She's more or less estranged herself from us now, and that has been pretty heartbreaking, but I've done my damnedest to ler her know that I'm still here for her.
We are still 'Mum', aren't we? Perhaps when your DD remarked in her pm to you on Facebook, that she didn't want children because her own childhood wasn't that happy, she was looking for a reaction from you?
I can imagine how wearing this continual negativity is for you, even though you know she can't really help herself. She most probably would benefit from some sort of counselling, but she doesn't want it. That's her choice. Would it maybe help you, though - if you could speak to a counsellor about how you're feeling about it all?