How to convince (or deceive!) my mother who is in early stages of dementia to get a diagnosis so we can begin to move forward?

Mom is in complete denial that anything is wrong, but it is obvious to her children and anyone familiar with dementia. She is still at a stage where she can do most things herself, but needs a lot of help. I moved in with her last year, but she wishes I hadn't. She is also narcissistic and the constant manipulation is wearing on me. (Although I try to tell myself it is the illness, I fight with her belief that nothing is wrong. I have my own illnesses to deal with and she pushes me into flareups everyday! )

I am not sure that a diagnosis would help me, but we can do nothing to plan for her future without her cooperation. How can I convince her it will help to get a diagnosis, or can I go around her to get one? Her GP is not helpful and thinks it is unethical for her to get involved.