I have an old dog in kidney failure too. Haven’t told her yet, she just keeps being happy.
I’m old too, and I’ve had animals my whole life, mostly cats and dogs in various multiples. Do the math and you can see I’ve been here before.
The way I reconcile it is pretty straightforward, and well in line with the overall Stoic approach to things. It always begins the same way- see things plainly for what they are, understand the natures of the things involved, and respond reasonably and virtuously to the reality around us.
Every day I care for my animals, keeping them happy, keeping them safe, shepherding them through their day with joy, and without harm. When they get old and approach death, nothing changes. As crazy as it sounds, the day I take them to the vet to be put down is the day that I have been working for all this time – I have successfully taken them the whole way. They did not get lost, they were not unhappy, they got to live their whole natural lives the way I wanted them to live it. We made it. We got there together.
When they are gone, my feelings for them don’t change. Their bodies are taken but my feelings are my own; I still love them, I am still happy to think of them, my heart is still open.
What has changed is that I have a space for another thing to love, and the cycle continues again, when I’m ready to start anew.
Their bodies, our bodies, everything external to us will always change and always come and go. Our love, our care, our joy belongs to us, and we apply it to what we have and to what is new.