My mother suffered from bouts of extreme depression from 1961 to 1983 when she died. She was treated with ECT and lithium, which brought on Parkinsons Disease and then dementia. She was doubly incontinent for some time and my elderly father was her carer for far too long.
Not surprisingly, it became obvious that she had decided she was no longer going to eat or drink, she had had enough. As she became weaker the care home staff called out the GP, who called an ambulance and she was taken off to hospital at 8.00 pm at night. I stayed with her, insisting on seeing the consultant geriatrician who was to be in charge of her care, whi eventually materialised after 11.00 pm. There was talk of putting her on a drip. I knew that meant she woud never come out, so refused all treatment on her behalf, and the ambulance took her back to the care home.
The staff kept offering her food and drink, but she steadfastly refused. Two weeks longer she went into a coma. My father was distraught, but I tried to explain to him that she had fought for so long and had now made her decision that she wanted peace. Now in a coma she was getting it, we were sad, but as far as we knew her torment was over, and we should be so relieved about that. She died peacefully a week later.
If your mother is depressed and the doctors cannot relieve it, then maybe she deserves a peaceful way out too. Eventually her kidneys will fail, she will slip into unconsciousness and ease into death. If you can, visit and talk to her, hold her hand, tell her you love her and hate to see her suffering. If you can effectively give her permission to go, you will both feel better about the separation.
Age doesn't really come into it, if she is exhausted mentally and physically, with unrelieved depression, wouldn't you want to spare her further struggles?
The mother you knew has changed, and naturally you start grieving for that loss, moving on to grieving for the physical presence. As others have said, it is a process that we all have to work through. I hope you have access to bereavement counselling both before and after the death, it really is valuable. You need to understand how to care for you too.