Mr Mitchell, 57, had said in June that his wife would need full-time care at some stage in her illness.
He added at the time that she often did not recognise their home and she has recently had trouble walking, prompting her to use a wheelchair.
This week he said: “She still seems unsettled and, in her lucid moments, I can see she’s worked out this isn’t temporary.
“She’s still thinking and saying I’ve let her down. Of course, that’s a painful thing to hear, but I know it’s not the Barbara I know speaking.