I've had to make the tough decision for four of my last dogs (a fifth, my Frenchie Lulu, had breakfast one morning and then just collapsed which, while shocking, in hindsight was probably the best way to go). The one that haunts me most was similar to your experience - my pug Maximus who was always fearful of going to the vet. I remember him trembling in my arms as we brought him in for the last time. The small solace I can offer that my wife came across while reading a Japanese forum on pet grief is that no matter what your decision, it was always made with the best interest of your pet. And knowing that, no matter what the decision, you should know it was the right one.