I can relate to what you are saying OP. My gran is 82 and is a second mother to me and I will be devastated when she is gone.
However, there is no point in dwelling on how sad that day will be as we have no power to stop it from occurring. We can only take comfort in knowing that we contributed to the aspects of their life which are good.
For Christmas I bought my gran a 'For my Grandchild' memory book and added a message of how much I love her and hope that she will complete the book for me so that nothing is forgotten about her. She loves it and it gives her something to do each day.
When can you expect yours?