What a tough moment in your life EE. The decision you make must be one you can live with forever. In your heart, what would she want you to do? She will love you always, but what would she want? I know she wouldn't want to cause you pain or sorrow. I feel the hardest thing on you is you must be the one to say goodbye. Of course you want her to live, but is it her time? What can you live with?
My support and best of luck with this. Hang in there, I know she'd want you to.