God, I know what you went through all too well. A few years ago I had to put down one of my dogs, Shadow. I was in my mid-twenties at the time, and he had been with me since I was about 8 years old. Minus a reptile and a hamster, he was the first true pet I had, and I literally grow up with him at my side. It such a cliche notion, but he really was family, really was my best friend.
I went away to college and obviously didn't get to see him much in his last few years. As extreme old age finally started to take hold, my parents called to say he had to be put down very soon. Being so near to graduating, I was unable to come home...Selfishly, I wanted to them to wait until I returned home a three weeks later - I wanted the chance to spend time with him before saying good bye. A day or so after telling my parents this, they called to tell me that he had suddenly improved greatly. His improved health lasted until I got home, where I got to see him act like his old self around me. It didn't last long though, and we knew it was time.
I only remember bits and pieces of being at the vet; it was such a traumatic experience. I remember holding and petting him as the doctor injected him. I remember the doctor being surprised that it didn't work, and at that moment I realized why he had miraculously gotten better when my parents were originally going to put him down - he had held on that last month of college so that we could see each other - walk through the woods together, play fetch (which always immediately turned into tug-o-war), sit in his favorite chair together while I pet him and read a book...(cynics can say differently all they want, but this is what I believe).
As the vet went to get another dose, I whispered in his ear, thanking him for being such a good friend...thanking him for waiting for me to come home...I told him that his work was done and he didn't have to fight it anymore. He looked up at me, and I could have sworn I saw him wag his tail ever so slightly, and then he was gone.
I will soon have to say goodbye to my other dog as well. She is 17 and though in remarkable health for her age, I know that blessing won't last much longer. She is every bit as good a dog as Shadow was, though they had very different personalities...where he was a calm/stoic dog, she is a rambunctious, goofy dog.
If I truly hate anything in this world, it's having to say goodbye to my dogs. Without question.