Have been going through the same thing with my Mum and demetia for the past couple of years, nutty. I find it helpful to just answer her questions straight up, no matter how often she has asked the same thing. When she is obsessing again and again on things that don't make sense, I sometimes enthusiastically change the subject to distract her. Don't bother correcting her, it won't work.

Perhaps I don't find as much of a sense of loss for some parts of her personality, as I have been in the position of mothering my mother since I was about 13. Some aspects of the adult-child relationship got a bit reversed early on. I probably won't know fully what kind of impact that may have had on me, until after she's gone.