Emps, so sorry. :(
My mum died of bowel cancer when I was 19w pg with DS; we only had about 8d notice that she even had cancer and that it was terminal (not sure whether she asked the docs not to tell us, I think she must have, because we got married when I was 17w pg).
It was a struggle to keep feelings under wraps through the rest of the pg, and because she was quite a public figure, we had a memorial service for her a couple of months later, which was hard but beautiful.
When DS was born, obviously I knew she wouldn't be around so I had asked my MIL to come over from Australia for the birth and afterwards - she stayed 8w in total, 2 before and 6 after he was born - and that probably helped relieve the feelings of "where's my Mum when I need her?", although I did still get the need to phone her sometimes. I dreamt about her a lot as well.
Have you any other female figures in your family who you could turn to? Of course I know it's not ideal, but it may help to relieve some of the feelings of abandonment (I know your mum didn't mean to leave you, any more than mine did, but it can still feel like that) - and even if not in your family, what about friends? How do you get on with your ILs?
It does get better but there will always be a sadness there that your mum missed out on your baby, and that you are missing out on having her around to help you. The sadness doesn't really go away, you just get used to it.
You will manage to face the ash-scattering - but don't look on it as a last goodbye. She's still with you in your heart, and (depending on your own levels of belief) still watching over you and the baby. The ashes are just the last remnants of her mortal body - and wherever they are scattered, she will be there as well - so you can go back (should you feel like it) and sit and be "with" her there as well.
My mum's ashes are buried in with her parents - I go to visit the grave every time I go back to the UK and have a chat with them all. Daft, maybe, but it helps me.
(((hugs))) - I know it's shit, really I do - but you can do it.