I feel for you buddy. I lost my Dad to cancer, so I can relate.
Your hurt will lessen over time, but likely never go away completely. Just be with her as much as possible now, have long talks, listen to her wisdom. Through you her legacy lives on. We all must come to the same end, eventually. It is hard to know there is no recourse but we all are living on borrowed time. Remember to take care of yourself (physically and emotionally), you gotta keep on keepin on bro! Cry with her, laugh with her, live with her now........