"I love you more than you will ever know"

This is a photo of my grandmother and me playing Christmas songs on the piano together- this photo was taken maybe 20 years ago?  Today, my grandmother is dying of cancer.  The doctors have put us on notice; "if you want quality time with her you better come see her soon."  Hospice will start coming to her house.

I saw my grandmother a few days ago, when she could still walk, and when I was helping her walk to the restroom I told her that I loved her.  She stopped, and looked at me and said she loved me more than I would ever know.  I left her house and went back to my parents house and cried.

How do I communicate all my love and appreciation to my grandmother without being sad?