Hi Tania,
sorry I have missed your posts, just caught up.
This is a lonely disease, there are so many problems with it and no drugs to cure the problems, it seems. Seeing your mum suffer like this is terrible for you, and for the many people who care for loved ones with Alzheimers.
I can't think what to say other than that you are doing your best and you must believe that. It is very wearing, but it seems there is little you or anyone can do to get it into your mum's head that she is loved and cared for.
Many of us know what this is like, I think you need to get all the support you can for yourself, your feeling bad and unable to cope will not be helpful for either of you, see if other people have suggestions, mine is to see your doctor first off, if you havent already, and to arrange for another day when the sitter comes.
love