I know how you are feeling, I just recently went through the same thing with my beagle, Snoopy. She was such a good dog, but someone just hit her and left her there to die alone. I remember when I was younger and our Airedale Terrier got hit by a car and I begged my mom to let me come with her and my dad when they put him to sleep so I could say goodbye one last time. It was so tough for me, and I will never forget the sounds he made or the look in his eye as he took his last breath, nuzzling his nose against my hand. That's one of the toughest things about owning a dog, though, you know there is a great chance of them dying before you do, unless you're an elderly person. Keep hanging in there, things will get better for you.