I am so sorry for the loss of your mother for you and your family. I lost my mother 5 years ago. She too had been sick the last 10 years of her life. The last 2 months of her life were very difficult for her and for us. As hard as it was then and still is for me, I at least know that she is now in peace and no longer suffering. I just lost my father 8 months ago. He had been ill the last 4 years of his life. The last month was horrible. But, he too is now at peace and reunited with my mother. This, to me is, at least, a comforting thought.
So, I know how hard it is: other than to say I'm so sorry. There are, unfortunately, no magic words that can be said to take the pain away. It does get better through time though to be able to talk about your mother and to remember and focus more on the positive things/memories that you have of her.
Blessings and healing thoughts being sent your way.