McS so sorry to hear you are struggling.
Was going to comment on the 'we're going to ... ' surely the only we here is you and your DH, but you have already realised that she needs to have a grandmother role, not a mother role, you just need to get her to realise this too. As others have said smile, nod, do your thing.
I too have times when I wonder what we have done deciding to have this baby, not because of problems but because I am selfish and wonder how it is going to change the cosy life we have. I do second those who have said to talk to a mw - even if it just lets off some steam.
Also push to stay in hospital. My cousin who is a nurse said to just put your foot down if you want to stay in and say that you have no one to look after you/no where appropriate to go. They can't make you go! Does DH get paternity leave? Could you both go to your mum's for the first 2 weeks?
Waiting sorry to hear your news, hope that it isn't as bad as it looks now and all gets sorted soon.