Something along the lines of...
I know you care for me, Mum. I know you want to be sure I don't run out of money. I really appreciate how much you show that you are concerned for me and your grand-daughter.
My life is not always easy, and sometimes, just once in a while, I want to build some good memories with my daughter, so that she can share them with you.
Please, let me build those memories without having them tainted by having a disagreement with you over this. It's something I'm going to do. I'm doing it for her, and I'm also doing it for me, because it is my 50th birthday and i want to have some positive memories.