I know, it's heart breaking, you know it's going to happen, you think you are prepared, it'll be just another smack in the face from dementia, but when it does happen it really is a shocker.
A couple of months ago mum and I were sat on the sofa, watching TV and chatting on and off, when she turned to me and said 'where's my son?', I said 'oh I think he's on a holiday abroad somewhere, not to worry your good old daughter is here' (my brother is an invisible, not been seen or heard of for coming up 2 years) she looked at me quite shocked and said 'I don't have a daughter, I have a son!', I replied 'I'm your daughter, Hayley', 'I don't know who you are' she said, 'your very kind, but your not family, it's not the same'..........I smiled, nodded and left it at that whilst something inside me broke a little bit more.....later the same evening, out of the blue, she turned to me and said 'What would I ever do without you?' I don't know if she remembered I was her daughter again at that point, I'd like to think so, mostly she knows who I am, but it comes and goes.
Take care of you