How to convince someone with dementia that something occurred, even though she can't remember?

Mom has dementia and can't remember things for more than a few minutes. She usually doesn't remember what she did or where she went the next day. She obsesses and worries about things and won't let up. The latest was her flu shot, which she got last week at the doctor office when she had a physical. She is convinced she didn't have the shot, and no matter how many times I tell her she did (I was right there when she got it and I explain over and over). I write it on the blackboard in her apt, write it on a piece of paper in her bedroom, tape it to the refrigerator. She erases what I write or writes over it "did not get" and writes little notes on pieces of paper. We have been over it at least 5 times in the past two days, and we think we have her convinced, she says I understand. Then in a few hours later, here we go again. I don't know what else to do, it gets her really upset, and we aren't able to calm her stress for more than a short time before she starts in again. We have been going through this over so many things, like she thought someone replaced her diamond with a fake, or she thinks she has a medical appointment when she went to it the day before, or other weird fears. Once she gets herself upset about something she won't let it go. Is there a drug that might help her? I hate to see her so stressed, hate getting phone call after phone call, and its stressful for the family. We are waiting for a room at assisted living as she can no longer live alone without some oversight. I know dementia can't be helped much, but I hate seeing her go through this kind of thing time after time.