Very sorry for your loss.
About to feel the same with little Benjamin-Tyree, cat, age 17+ who has maybe days or a week or so, before he'll take that last trip to the vet.
And, having lost five of my "kids" now, I still wonder how it is that we are, as we are, and why such little lives of joy are so short. Over and again, I'm brought to believe that it is by the design of the Almighty. And without meaning to sound too preachy, I shall expound a bit.....
I think the condition that we have, with animals that become a part of our family, are by design. They serve to illustrate, in many ways, the aspects and principles of life for young people (and old alike) who form a relationship with their pets. That is, they are allowed to see and experience creation, care, nurturing, love, hope, despair perhaps, and even loss and grief at the time of the pets passing - all serving to illustrate lessons of life, but even more so, serving to illustrate the relationship our Creator has with His creation. When our pets pass, we may be reminded that we are we, but He is He. The Almighty is the Almighty, and life is his to dictate and ordain. If we should feel grief, remorse, maybe guilt, if as humans, we do the "humane" thing and end the suffering of our pets, think again of how we, as humans, are to the Almighty, who controls all life, when we, His most personal creation, pass at His will.
In a way, knowing how powerless I am to control anything really.... and how ordered the universe is..... is humbling. I'm reminded of that all the time, but especially when someone dear to me, even a pet passes away.
Very very sorry for your loss, your feelings are not alone.....