My mom depends on me for everything. I take her shopping, to all medical appts, I fix her computer, TV, etc., etc., etc. It is very odd that sometimes she will tell me about the "helper" lady that stopped by over the weekend and fixed (or broke, depending on what is going on) her computer. The "helper" lady is me. There is no one else. Yesterday she told me how "Betty and Fred went out for their anniversary on Sunday," I am Betty. Today when I called to tell her I would pick her up at 4:00, instead of 4:30, for grocery shopping, she got upset because "her youngest was picking her up at 4:30". It goes on and on. I know she isn't doing it on purpose. I know she doesn't have bad intentions. But I get so frustrated with it. Today I just abruptly said, "This is the youngest, mom. I will pick you up at 4:00," and quickly got off the phone. My sister and nephew live 9 hours away so no one else helps her. It is very weird and sometimes I just don't want to hear it or deal with it. Sorry to vent. It has been a weird couple of days.
As for your frustration with her, that's perfectly normal for a person taking care of someone with dementia. When someone pretty much has an adult-sized pre-schooler that they have to do everything for who will only get worse, it is very frustrating. It can make a caregiver very angry and is also very sad. Be watchful of yourself though because it sounds like you have compassion fatigue aka caregiver burnout.
You need to get some help for your mother or put her into an appropriate care facility where she can be safe and properly looked after. Stop trying to do it all on your own. You love your mother, but when a person is burned out on caregiving and doing every little thing it can lead to depression, anxiety, and even elder abuse.
That being said, caring for someone with dementia is very stressful. She will get worse and there will be more demands on you. Perhaps it is time to consider help or facility care, even if she doesn't want it. The demented person cannot make good decisions. Tell her this is help for you. You have to run the show now. This is not about her wishes, but about her needs. And also about your needs. The caregiver matters too.
When their brain gets to this point, you don't know what she might come up with next. You can't know what she's coming up with when you're not there. Or what her thoughts are.
My elderly neighbor was like your mom. Her daughter thought it was just a touch of confusion, but one fall evening her mom took off all her clothes and walked naked down the street in the dark. (And the daughter worked at a memory care facility, so she should have understood the problem before she did!)
Your mom clearly has dementia, and no burying your head in the sand will stop it. I'm sorry.
I would stongly advise to hire (with your mothers funds) an aide to help with daily responsibilities like shopping, house cleaning, help with laundry.
It may be time to have her move to assisted living or memory care if she has a dementia diagnosis if she is living "independently".
Your wellbeing is your first priority. Do not neglect your physical or mental health at the expense of your mother's intentions of staying in her home. With the longevity and advanced medicines and healthcare it may not be sustainable for the elderly to die in their homes as it was years ago. It will be detrimental to you in the longrun.
Good luck. I wish your strength during this difficult period.
Best of luck to you.