I am so very sorry for your recent loss. I too felt I had to make that final choice and end my beloved's pain & suffering. It was believed he was around 13 years old. He was an orange & white Tabby cat named "Marmalade." Although he was deaf, and had a history of chronic health problems, from having been a stray cat until around 9 years old, when I started to try and help him, he was noble & true. He had been the KING of a colony of feral and stray cats in New Mexico where our paths crossed over 850 miles away.
There is a saying shown below, that has brought me some comfort and helped me to heal somewhat. I hope it comforts you too and helps you to heal:
"When we show our pets mercy, and agree to end their pain & suffering, we than take their pain & suffering onto ourselves. We transfer it to ourselves. We absorb it. And then we process their pain & suffering through our grief. This is the bargain we make."Kind regards & my sincerest condolences.