That sucks. About 1.5 years ago, my dog got a subdermal hematoma that the vet didn't quite fix properly. After a lengthy healing process (like, 4-5 months) he was almost home-free - the hematoma in his ear was just about healed, everything was fine. Then my dog lost the ability to walk with one of his hind legs. Turns out he had cancer ALL down his spine, and it'd been there for quite some time. So we had to put him down. I still remember being there with him. It sucked. Balls. He really was my first, unconditionally loving, friend. Lol, in grade eight I made a plaque in my shop class that read "Matt and Symon's Room". Lol, I still have it.
I got REALLY depressed after it happened. While 'technically' he was the family dog, he was really my dog; he slept with me, followed me around everywhere, and was very protective of me.
I know this sounds wierd, and probably not the best thing to do, but *shrugs* it might help. Play some video games. Seriously. Pick a really involved, long, drawn-out video game. My brother and I played World of Warcraft almost 16 hours a day for about 2-3 weeks afterwards, and it actually REALLY helped me get out of my depression. When I was playing WoW I was concentrating on the game and not on the situation. It got my mind off of it, and the initial shock of what had happened started to wear off and I could accept that the decision was the best one for him, and while it still sucks, and I still miss him...it's not dominating my life.