I've lost dear pets also. Childhood companions that by their nature can't join you into adulthood. SO I know somwhat how you feel. Giving the animal peace is important when the time comes. I've seen some animals who have gone into a state of degeneration that is no longer beneficial to owner or animal. It's sad to watch.
I hope you enjoy the good memories and hope you remember that it was an act of love to let the dog go.

Take care

Steve