...when I suggested that she'd hidden her purse again and forgotten where it was, after she said she couldn't find it.
She thinks too that I've given a key to a woman who comes in when we're out and steals her things, and hides soiled underwear under her mattress.
What to do?
Are you living together sandy? You must get professional help. One person alone cannot deal with this, which I assume is either dementia or something similar. I think you must ignore what she says and see her doctor. She can stay at home with help until she gets too bad but there is help available.