I get where you are I’ve been in a similar position before. I was extremely close with my grandma. Honestly the most important thing to do is keep the relationship strong. Call her, write her, try and see her, make sure you say “I love you” and tell her how you feel. Getting that phone call is something that I’ve been through and it is awful. But the most important thing to remember is to focus on life and her RIGHT NOW. At this point in time you are so lucky to have her, and appreciating every single moment and trying not to think of something that is so unknown in the future is key. Live now and you will handle everything that may happen correctly in the future. Sending my love ❤️