My great grandmother actually recently died from a long battle with Alzheimer’s. It’s an incredibly difficult thing to watch someone slowly lose their mind. Saddest point was when she could no longer remember who me, my siblings, or my parents were. She would freak out any time we entered the room because she thought a bunch of strangers had broken in.
It’s hard, and the only advice I can really give is to do anything you can to be patient and understanding. I know it is frustrating and she may be annoying to deal with at times, but push through it and try and enjoy the time you have with her. My one regret with my grandmother is I started avoiding her, and I wish I had spent more time with her while she still saw me as her grandson.