Dementia Support Group
Dementia is the progressive decline in cognitive function due to damage or disease in the brain beyond what might be expected from normal aging. Particularly affected areas may be memory, attention, language and problem solving, although particularly in the later stages of the condition, affected persons may be disoriented in time, place and person (not knowing who they...
My mother also sometimes gets fixated on something as part of her advanced-stage Parkinson's Disease. I, too, have done all I can to rationalize with her when she gpes down such a path. Often it has helped, but sometimes it doesn't. The best thing to then do (for her AND for you) is to assure her that what she believes is being done is not so and then follow it up with something completely different to redirect her attention, something non-stressful.
As my local Alzheimer's group advises its caregivers, "don't try to rationalize with an irrational person"....or, in my case and perhaps yours, with someone who, from time to time, is fixated on something that is inaccurate.
Also do some research into your mother's medications, which can contribute to or even cause obsessiveness tendencies or even paranoia. Some medication dosage "tweeking" or a complete change, if possible, may make a big difference.
Sending you my best wishes. This is difficult stuff, for sure!