None of these is exactly what you want but they are as close as I could find. I share your experience of never having known two of my grandparents except through the memories of others. Good luck in your search.
Parting is all we know of heaven and all we need to know of hell. ~Emily Dickinson, "Parting"
Your absence has gone through me
Like thread through a needle
Everything I do is stitched with its color.
~W.S. Merwin, "Separation"
Every parting is a form of death, as every reunion is a type of heaven. ~Tryon Edwards
You left, and my heart is a ceaseless sermon of loneliness. ~Jaesse Tyler
The reason it hurts so much to separate is because our souls are connected. ~Nicholas Sparks, The Notebook
" I don't miss you and you alone, I miss you and me together."
Unknown
" There is one pain I often feel, which you will never know. It's caused by the absence of you."
Unknown
Pain is inevitable, suffering is optional. M. Kathleen Casey