Yes sometimes we have to make the big decision and it is hard to do, but we must think of the poor animal and the pain it is suffering. My old cairn Hamish was 16 years old too, his back end collapsed and he had kidney problems and it was a struggle for him to move. So we did the kind thing and put him out of all his pain. It was very hard for me but the right decision was made and he died in peace.
Oh mine to , it's so sad , but as you say the animal is the most important , and that they don't suffer, it's the hardest thing to do