I can relate well to this predicament too. It's very hard to keep away that emotional suffering you know is inescapable. The way I dealt with it with my last dog I had who died of old age was to get a puppy a year or two before it was my beloved Shepherd's time to pass away. My Shepherd has now been gone for a couple of years. I think that the effect of developing a bond with my present dog while the other one was rapidly aging caused me to have an even closer bond with the Chi-Poo I now have.
A dog that loves you deeply suffers less going through old age when s/he can sense you're doing what you can to continue life without her or him. Dogs don't get jealous like people do. When they love, they love unconditionally.
Love means when you're sad, the object of your affection is sad too. Maybe realizing that will help you to know how important it is that you focus onto grabbing what you can that brings you peace and comfort. Because of knowing that having a young dog means many years ahead of wonderful companionship, it helped to counterbalance the grief I had while my older dog was aging.
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"Has not my hand made all these things, and so they came into being?" declares the LORD. "This is the one I esteem: he who is humble and contrite in spirit, and trembles at my word." – Isaiah 66:2