I'm very sorry for your loss, and for the pain you are going through. I can appreciate the shock you feel at the speed with which this disease claimed your grandmother. I still am numb over the loss of my dad, and its been 4 1/2 months. I hope that the memories that you have with your grandmother will bring you some peace during this holiday season.
I'm passing along some advice that my cousin gave me after dad died...and she still reminds me of it periodically now> breathe. The panicky feeling will creep up on you for awhile, at least it does for me. Just remember to breathe.
You and your family are in my prayers.
"Faith in something greater than ourselves enables us to do what we have said we'll do, to press forward when we are tired or hurt or afraid, to keep going when the challenge seems overwhelming and the course is entirely uncertain." ~g. b. hinckley