Certainly one thing she needs to hear from you is that you love her, and that she isn't a burden but a blessing to you. After all, you'd rather have her with you than to be without her the rest of your life.
I honestly don't know why God allows some people to be taken by death at an early age, while others must struggle with the infirmities of old age. But I do know this: God loves us and knows what's best for us, and even when we can't see the whole picture, He does. Perhaps God allows some people to live longer simply to remind the rest of us that some day we too may be weak and frail, and now is the time for us to live for God.
Do all you can to encourage your mother. Be a good listener and hear her complaints -- but remind her also that God still loves her and is with her, no matter what she faces. God's promise is true: "Even to your old age and gray hairs I am he, I am he who will sustain you. I have made you and I will carry you" (Isaiah 46:4).
Most of all, remind her of the hope we have of heaven because of Jesus Christ. By His death and resurrection He conquered sin and death and hell, and when we put our faith and trust in Him, we know that some day we will go to be with Him through all eternity. Does this hope live in you?