I've had to put down a few animals that were in pain. They were going to die anyway and I couldn't bear to let them continue in their agony for however long it would take them to die. None were pets, but every one of them still bothered me. I know it was the right thing to do, but I can't shake that feeling. Like you said, when I think about them, it's like it happened just a few moments ago, raw and intense sorrow. Some I didn't feel the feelings at the time, but I feel all of them now.