First, I am so sorry that you are in this situation. My MIL just passed from dementia and it is a tough thing to deal with. We now feel my FIL has it...
Contact your local alzheimer's association and get some help. You certainly are not alone in this. You need to get legal documents in place if they are not there so you can take care of here such as durable power of attorney and living wills. She needs to be evaluated by a doctor who knows dementia and who may be able to suggst things to help her - and it can be things such as thyroid, vitamins - in other words, reversable and treatable things that can cause dementia - or it can be the real thing. So she needs a real good physical. It is hard that with the personality changes, all becomes confrontational - it is that way with my FIL. They get very fixated on strange things and night and day get reversed and the anger is something else.
Get her to a geriatric doctor, get a lawyer, get help (you need it), get advice, and take care too, of yourself - as this is very.. well, draining.. like you said - this is not the mother you know. Sorry to say, she does not know herself either.