One tired nanny

You have already started a thread on this twice before I think. It would be better to return to you original question,consider the responses given and continue the conversation there.You can always update as you have done here
Well done to you for the support you are giving.

I think you are looking for a GP who has full custody of a GC to maybe share experiences?
I have looked after my GCs for 3 days a week when they were younger and I wouldn't have wanted full responsibility for them. Teenagers can be difficult and I can understand how tiring they can be so I admire you for taking this job on.
The only comfort I can give is that they do become more reasonable as they get older 💐

No I haven't

But I think you should go to citizens advice and find out how you can claim financial help and responsibility for your grandaughter in order to get the right to speak on her behalf as a guardian.

I think you need to approach your GP and find out how to get support for yourself.

And I also think you need to approach the school and find out how you get help for her in the form of counselling to help her adjust to her new life and how to deal with whatever caused the relationship with her parents to break down, or whether leaving that relationship is what is actually best for her.

VioletSky* gives good advice jodyjay. I'm not sure why you have started a tgreads on this topic but I do think that VSs suggestions might be a way forward for you. It must be very hard for you and your GD.