This is a very difficult situation. I'm assuming your mum is currently living either with you or on her own.
There is an almost inevitable path of progression whereby the person with dementia slowly loses their grasp and eventually needs full time care. It sounds as though your mum is well down that path. Trying to look after her yourself will mean buying into potentially years of strife, whereby your own life will be put on the back-burner. It's a relationship destroyer and far more stress than anyone should be expected to endure.
There is no easy answer. Generally I believe that re-orientation and trying to argue that the person's view of reality is wrong, is largely a waste of time. In my experience it's far better to go along with the delusions, so long as it's not causing harm, as even if you manage to convince her otherwise, in the moment, minutes later you will be faced with having the same argument again.
Diversion and delaying tactics probably work best. If she starts kicking off then agree to take her home to see her parents, then drive around for a bit until she calms down. Take her back to your place saying "just need to pick something up first", but then delay and divert onto something else. If her memory is that bad then it will probably work, sometimes. If she persists in her delusion and starts getting upset then medication is probably the only option. You just need to constantly divert as you will never win and convince her otherwise.
Longer term, perhaps you could talk to the GP about medication that will keep her baseline agitation levels lower - basically doping her up a bit. It's harsh but faced with the behavior you are seeing, perhaps the only option.
You also need to seriously consider long term care. Maybe not to be implemented immediately, but you need to get yourself in a state of mind that you are okay with the decision. It's a hard thing to do, although most likely the right thing. You need to ask yourself, would you want your children to look after you if you were in the same state? The answer is almost inevitably no, and neither would your mum want to be a burden to you.
I'm sorry if any of this comes across as harsh or dispassionate. Dementia is the most terrible illness, in my opinion the worst, and by its very nature is cruel and upsetting. There are no easy answers.