I never had an experience that bad. I had a vet come to the house for CeeCee in December. He had put down some of my goats, and is very experienced and gentle. It took her a while to go down after the first shot, then her veins were collapsing for the next step. He had to shave, try several locations, and she did bleed some, but she was comfortable. The cost was $600 which included cremation. He also came out on Labor Day to remove Mickey who had died in his sleep. I think that was $300. I had to take Ace to my vet in February. It took him a long time to go down after the first shot, but the vet just gave us private time with him. Although he kept walking around, he was not in pain or upset. The final shot was easy. He was very out of it by then. I don't remember the cost, but it was several hundred less than the at home vet. In November, I took Misty to the specialty hospital that treated her frequently. She loved going there, so wasn't upset. She had lost twenty pounds and was very weak, so she just slipped away. I e taken several other dogs to my own vet or emergency hospital, but never had a bad experience.
I agree that you probably won't forget what happened. When I had a horse put down, we walked him to the front of the house to make removal easier. The guy showed up with the truck to remove him just as the vet arrived. He stayed in the background, but it was uncomfortable. The horse was not upset, and he did ok with the first shot. However, he remained standing even after getting the "pink stuff". I left to go in the house while the vet and the guy pushed him over, but I'll never forget that thud when he went down!
All animals are different, and we all hope for the best in that horrible situation. I'm sorry that you had to go through it, but there is nothing you can do to prevent it next time. Maybe mention it to the vet at the time to let them know your concerns?
Hugs!