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.
