I'm really sorry about your loss. We had to put our dog Scooter, about the same age, to sleep. He was actively dying of kidney failure and having a really hard time with it. His death was very peaceful. We founded a moderate a small youth group and Scooter was our best attendee! Evryone wanted a funeral so we built his coffin for a group project and had a "wake" and attendance of about 18 people.
Then a young man who had a tbi in 2002 that I assisted with that and a host of other medical and social issues died very suddenly and unexpectedly of an enlarged heart that his doctor did not know he had. Very sad for me and my family as we had all become rather close. He was only 39 and a single dad of two. That's how I'm feeling right now, rather numb.