Mom's delusions that are positive (my grandmother owned the state of Illinois, her uncle invented the airplane) are easy to ignore or play along with. The negative ones (I have a drinking problem, my husband cheats on me in the first not yard every night or her latest that she is firing of cerebral palsy and if I don't take her to the ER she will die within 24 hours and demands that I call her home health nurse) I have difficulty redirecting her in a positive way. She refused supper once due to this ( she said then God told her she had Heteroptera, a disease apparently know only to God) and almost didn't eat last night (until she found out it did not include lettuce that gives people cerebral palsy). Is there a way to ignore or redirect these negative delusions without causing her more distress?
I think this may be a function of the nerve synaptic breakdown that occurs, when thoughts aren't completely or logically formed and tend to be truncated so that they really don't make sense logically.
The only thing I could suggest is (a) re-examine the medicine regime and determine if there are any interactions, and (b) go along with her and try to make her feel comforted, even though she may not be able to understand what you're saying.
E.g., as to the back yard cheating, perhaps you can just go out in the back yard (if she's mobile enough) and look for him. When you don't find him, that MIGHT allay her fears but I really suspect there's not much that can b/c of the way the nerve connections are damaged or broken.
Maybe you can find something else she likes to do, or put on some hand clapping music to overcome her fears.
My cousin, who has severe dementia, at one time was very concerned about various things. One was that her cat would escape from her home through a crevice the size of a pea. Another worry was that a certain staff member who worked at the AL was okay. She worried about him and looked for him constantly, looking out the windows, biting her nails, etc. Nothing could convince her to relax and that he was okay. Medication really helped with that. She is now content and isn't always worrying or biting her nails or looking for people. She is no longer overwhelmed with concern over certain people's welfare.
Now, if she occasionally shows concern over something, (she can't say what it is, only that there is a problem and she just seems worried), I tell her that I have already taken care of it, it's solved and that there is NO MORE PROBLEM. (Of course, I make sure there is no actual problem first.) And that I'm there to celebrate with treats. I always take her favorite treats, so I have them on hand. That makes her feel better, relieved and we move on.
However, if the trouble your mom's brain is set in a certain way, I'm not sure if verbal assurances alone will dissuade her fears and beliefs. I'd explain that to her doctor. Get his opinion and/or a referral to a geriatric psychiatrist. This must not only be very disturbing to the family, but also to her, because it's real to her.
I feel for you, these are the hard parts of caregiving that just don't have a chapter in the "how to" manual. Sending [[[[hugs]]] your way, they won't fix it, but hopefully they remind you that you aren't in this alone!