I know how hard this decision is for you. I've been through it too many times. We owe them to let them pass with their dignity and I've made that promise to all my Labs over these many years. No matter how hard it was for me, I always made the decision as to what was best for them. Someone here once made the statement, "better a day to soon, than a day to late".
I truly feel for you, and hope and pray for as many days as you can get. In the end, it's never long enough, but always know they will live forever in your heart.