I'm so sorry for your loss. I know how heartbreaking it is to say goodbye to someone that you consider a full-fledged member of your family. I had to make the decision to put down our family miniature schnauzer after an incredible 18 years of life.
It's a shame that dogs don't have lifespans like humans so we can spend our entire life together.
But anyone who keeps animals knows that dogs love more in 10 years than most people do in a lifetime. They are love. That's what a dog is.
I may be the only one, but I believe that you'll see your dog again someday. The truth is nobody knows for sure. 15 years is a great life.
Thoughts and prayers are with you and your family.