V.W.
I cannot offer much, except this:
Please be careful about how you express these feelings in front of your granddaughter. She is going to have a hard time adjusting to all the changes in her life, and she needs to see positive happy thoughts of her future, from you. Not a grandma in tears over the prospect of her moving away with her mom. Think of what a blessing this move might bring for both of them and speak positively of her move and new home.
If you focus on helping your granddaughter through this by being a positive influence on how she accepts this move, then perhaps it will help you cope also. :)
Tell her how you will spend your time and that you will send her pictures often of what you and her granddad are up to, and then follow through. Ask her to remind mom to send pictures of her also. That you'd like to see her in her new room and her new yard or at the park near her home.
You'll get through it.
God bless you.