I'm sorry.
I remember when my first dog had cancer, and there was a terrible accident and he was suffering too much. My dad had to take him in the back and shoot him because there wasn't enough time to take him to the vet. I was very, very sad.
I have lost a number of pets over the years, and it's never easy. My house isn't really a house without a dog, or even a cat... I can't wait to have my own again in the future.