I know what you mean. Having to put one of my cats to sleep, she had kidney failure and had fought it for a year, was the hardest thing I ever had to do. Up until that time, I had always felt it was up to God, not me, to end a person's or animal's suffering. I experienced her battle with her for that whole year, giving her IV fluids through her back every night via a needle through the tuft of her skin. She was always so patient and understanding, laying in my lap, knowing that the pain I was inflicting upon her was done out of love. I came to a different understanding about life and death through that experience. That morning, she told me she was ready, and I knew putting her to sleep was the right thing to do. She seemed to know that this visit to the vet would be her last visit, and she wouldn't have to go through that anymore.
I'll never forget it. I called my dad that morning to drive us to the vet's office. (My dad has since passed away too.) I just couldn't drive. And I knew I needed to hold her in my arms. When we got there, the vet's staff was waiting for us. We were taken to the back of the clinic right away, no waiting. We laid her on the table in the room reserved for the worst cases. I had her favorite brush with me, and I brushed her over and over and over... No one rushed us. Then the vet stepped in and asked if I was ready. I said, no, of course, but yes, let's proceed. I got down to her eye level, hovered over her, and continued to stroke her. He injected some medicine that would make her fall asleep. Then, as we had prearranged, I would leave the room. I couldn't stand to see the final shot. I gave her one last stroke and a kiss and told her I loved her and that I was sorry. My dad and I stepped out of the room. The door closed. My dad held me in his arms. Then a tech opened the door just at the moment the vet inserted the final injection (blue juice they call it) into her, and I looked over, out of habit as anyone normally does when a door opens. And I saw it. I felt my soul leave my body, I no longer could stand on my feet, and uncontrollably, I fell to the ground, literally shaking. My dad caught me in his arms just before I hit the floor. I will never forget it. That feeling.
I hope that was okay that I shared this here. Seems to be the perfect thread for it.