I am so sorry to hear about your loss.
What everyone has said is so true. I had to put my dog duke at a young age due to cancer, he was the first dog I had ever had picked out as a puppy. It killed me inside because that was my best friend, companion, rock to cry on, ears to listen to my problem, tear licker and the like. I kicked myself every day for a year after I had put him down. Depression was the worse.
Finally I realized, on the outside it looked horrible, putting my best friend down. Then i thought to myself, would I have been a better person if I didn't? Him never gaining weight, loosing it when he ate, loosing his fur, pain just not himself anymore. I would have felt worse to put him through that so I would not have to go on without him. So now I keep a picture of me and him above my computer. From time to time i break down and cry because I think of him, but i remember how much i loved him, and he loved me and I gave him the best life in the time we had. I also have a picture of him in my wallet, I feel it is like having him with me all the time, besides being in my heart, if i want to see him I just open it up if im out and about.
It is hard to get over and we do never forget them. We move on and keep them within our hearts till our time comes and I truly believe, our friends will be waiting, all that we had lived with and loved through out our own life. I believe that is just how loyal they are.
I am sure your boy is happy you did this for him, that you had the heart to help him cross because of his pain, and gave him the relief of it. I am sure he is watching over you from above and thanking you for it. To give him the freedom of being painless. I am sure he will always be grateful and what I think makes him more happier, is being there for him when it happened. It shows you loved him so much you had to be there to comfort him.
Some heal and cope faster then others, and some find comfort in rescuing a needy soul some what soon after. But I am sure when they time comes and you do find another puppy/dog who steals your heart and needs to be saved, Kirik will be "cheering you on in the doggy way" for saving another soul to love and cherish just like you did his.
It is good to talk about it, get it out. I feel it does help so much and there are so much wonderful people here who will listen, and know the pain.