Oh wow, that is very tough and you have my sympathy.
I wish to relate a story, I hope it is a bit of a comfort. MY grandmother spent her last years in pain and constant surgery. Near the end she was basically unable to make her own decisions. The doctors contacted our families, and there was a big meeting. They decided that she had suffered enough, it was her primary wish to be released from her pain. The doctors wanted one more surgery, and it would have accomplished little but prolong her pain. The family said no, no more constant surgeries and pain.
Sometimes our personal powerful emotions not to let go of a loved one clouds the big picture, and the big question. Not what is best for me, but what is best for her.
We never want to let go, we want them in our lives forever. But you must search hard and determine what is best for her, what she wants.
Yes, life is cruel, one can never avoid that basic fact. All we can do is endure, keep our heads up and appreciate our loved ones as much as we can, each and every day.