A very dramatic time for you. Have you asked any of the nurses on duty, or even one of the doctors who help treat your mum how she is when you aren't there?
You may find that although she is incredibly upset when you are visiting her, because she expects you are there to take her home, or that she can at least ensure you do as you are told, she may well be sweetness and light, or at least resigned when you are not there.
Main thing is don't let it make you feel guilty, I know that's easier said than done, but it's not your fault. If your mum seems to be coping better when you aren't there, is it possible to get someone else to do the odd visit, who isn't quite as closely involved, such as a friend if she has any left, or one of her neighbours if any of them have known her for some time, and have been friendly to her?