Exactly kitty - feel sad, by all means, but do not feel guilty. I was sad when Mum died, and the fact that she had been hard to get on with compounded that, but their was no need for guilt.
It is very emotional when we have a concept/dream of how a mother should be, but we have not been lucky enough to have that in our lives - it feels so fundamental. As you rightly point out, she would never have been that person, however long she lived.
I feel sad that my mother's life was blighted by the huge chip (more like a plank!) on her shoulder about being a woman in a man's world. It made her very bitter and she took it out on my Dad in a big way. I feel sad at the wasted opportunities for her to be happy that this caused. But I cannot be responsible for that, any more than you can be responsible for the difficulties with your Mum. She is as she is, and, in spite of all, you are visiting and overseeing her well-being - so well done you.
It will be hard when she passes, but you will come to terms with it - I did and I am sure you will.