How can I say no to my mother who is in a nursing home?

mother has been in a nursing home for about a year now. she has broken many bones, hip, leg, ankle etc. She has been in a wheelchair for many years, but did manage to live independently. She had hip surgery last nov. when she contracted c diff which she has never recovered. She has multiple medical problems, which I won't go into. She is a widow and lives just across the way from me. I am the only caregiver. She is determined to leave the nursing home and come home. My mother's mind is still sharp as a tack, it is just all of her mobility is gone. I work 12 hours a day and cannot look after her. The doctor said she needs 24/7 care. What can I do? The care she needs would cost more at home than the nursing home. How can I make her understand and keep my sanity? Any help is appreciated.