My mom who has dementia is depressed, what to do?

Hello,
My mom has dementia. She is getting more depressed as time goes. She recalls more and more situations incorrectly, and puts many wrong details into them that make her very upset. She refuses to see a doctor, always hated it, doesn't and won't take any medication (has not been prescribed any). She doesn't deserve to feel this way, and has helped me many times, so I'd like to help her. For instance, my sister and niece just left after a very nice visit. My mother is now saying that my niece took her gifts out of the closet without asking, is an ingrate, and that my sister should have paid for lunch when my mother fought to pay for it. It was the only thing my sister let her pay for, and we went to extensive links to make my mother feel as though she paid for something when she didn't actually do it.
Additionally, it is very different from my mother to fixate on something negative. She is always the one to say to look on the bright side. I know you're not supposed to change the reality of somebody like this, but I tried to correct her perspective to help her feel better. That was just as bad or worse, now she doesn't even want to talk to my sister.
Many thanks in advance!