I read the OP in the early hours and couldn't make a suitable reply, I find your situation very sad. Having been in simliar situations you learn that life is a bitch and we have to do what we think is the best under the circumstances.
We make promises to reassure and those promises must change as each situation develops.
I will never forget making exactly the same promise to my parents when they were in relatively good health. When mum finally left hospital for her last Christmas at home she was too ill to get out of bed. When the ambulance took her to the Hospice she said " This is the last time I'll see Broomfield Road, I won't be coming home" She was right but the care she received was excellent better than she would have got at home. In fact I lost my dad 18 months later and he asked to go to the same hospice, I could keep that promise.
We made the same promise to Cheryl's mum but dementia was making her think that she wasn't in her own home anyway, again she was in excellent hands at the care home.
We can't beat ourselves up over broken promises but take solace in the fact that we made the right decision at the time.
I wish you and your family well for the future, you can only do what you think is best and I'm sure you will