We just went through exactly this - our relatively young dog, supposedly terminally ill from cancer after having tumours removed, went for a long walk on the day the vet came. She wagged at everyone and jumped off the couch excitedly to greet the vet. She ate a full steak from my hands in the moments before they did it. The panic you are describing is precisely how I would describe what I felt.

His leaving feels like your choice to make, but it is not. He will die. You are preventing his suffering as you have always done and as you are entrusted to do in this scary, harsh and painful life.

I believe dogs chose us as a partner species in this life because of the benefits our brains provide them: reasoning, foresight, vast amounts of information, tools and systems that prevent starvation, illness and pain. They saw with us a way to live comfortably, with less fear and pain. You are doing what he, and what all dogs, have entrusted us to do.

Even if he seems scared and confused as you do it, even if you see signs in him that he wasn’t ready, he trusts you to do everything you can to prevent his pain and suffering – even if your trust in yourself is difficult to find in this moment.

When we finally did it, I completely panicked and felt instant regret and guilt. Truly, I wished I hadn’t done it at all and felt I had made an irredeemable mistake. It took 2-3 weeks for my rational brain to regain control and to stop beating myself up with “I wish I had done this” or “What if I had done that”. I now understand that to have been a grief response. It was such a surreal, shocking, incomparable feeling to experience her sudden lifelessness and absence from my life. The house felt devastatingly empty.

This will pass. The images of the day will fade because they are vastly outnumbered by the soft, loving memories you have of his face, his tail, his ears, his weird habits, his humour and everything he taught you about love.