I believe its because she has dementia that this has been allowed to go on for so long.
My grandmother died in the same way last year, but it took almost 8 torturous months for her to finally leave her body.
She was the kindest, funniest woman I've ever known. I was lucky enough to have a Granny well into my 40s though since covid and being locked away in a nursing home her dementia had become so much worse she had days were she no longer recognised us.
As she could not speak for herself and say I do not want to eat/drink the nursing home had to keep attempting to give her nourishment- even if it meant it was 1 sip of water or a bite of food a day. Just that kept her sustained for many many months after we were told she was dying and to come and say our goodbyes.
Conversely, at the same time my cousin who was dying of cancer but was mentally healthy decided he wanted no more treatment, no more IVs, food or drinks and was allowed to pass peacefully.
Granny just kept going in a manner that was torturous to her, though thankfully she was unaware, each 15 minutes seemed to reset in her mind. It was painful for me as her granddaughter but it nearly broke my Mum to live with the pain and responsibility for 8 months.
I'm sure the pressure you are feeling is enormous and the fact that you feel like you are grieving a person who is still here is very hard to untangle. I wish your Mum a peaceful passing very soon and that you can finally get some rest in both body and mind.
It's probably too late for you as your mum is now so ill and cant consent. I know we couldn't get it here in N Ireland. But if you can try and get medical power of attorney for your loved ones and then you can decide if they can be fed or given drinks etc. I believe this goes beyond the powers of a DNR.