Mom had a car accident in 2011. Since then she has not had a car. It was a total loss, no license, but for 5-6 days she's demanding to know who took her blue car because the tags are due. She's never owned a blue car. I tried the "it's in the shop, I'll get it tomorrow" but that leads to, let's go get my car the next day! I tried the whole accident story and that leads to same question 4-5 hours later. I tried the you didn't own a blue car and that just makes her mad. Actually whole topic is making her mad. Last statement was "You're just trying to make me seem crazy and keep my car". Any other ideas? She's usually not so angry or obsessed. Things don't usually carry over for days. Most of her faulty memory is founded on truth never something like a blue car she never owned. Nobody in the family owns a blue car either.
When my LO was in that stage, it seemed that it went from one thing to the next. There was never a time that there was not something at issue. Sometimes, there are no real remedies, just repeating things over and over and trying to keep them calm and as content as possible.
The father of one of the caregivers in my support group had very persistent delusions he was obsessed about. They went on for months. He thought he owned a whole town -- and talked about what he was adding (a new bowling alley) and what needed attention, etc. Then he thought he had been elected governor (while still retaining the town delusion). Mostly these delusions weren't terribly hard to go along with. "Oh, a bowling alley! That does sound like a nice addition. Will food be served there too?"
The problem came when he demanded some action. He had to get to the train station to give his inaugural speech. "You have to help me pack and take me to the train station." Sort of like your mother insisting on going to the shop to pick up the car, right?
Does it help to know you are not alone? Probably not. If you figure out something that works, let us all know!