Pets are like family so I can only imagine what you're going through right now. Sixteen years is a long life for a dog, so you must've been a great Dog Dad for your dog to live that long.
My pitbull will be 12 in August (if he makes it that far) His back legs are weakening with hip dysplasia and he has masses on his spleen. I will be a basket case when I have to eventually put him down...