I had the same thing happen with my 14 year old dog last month (actually it's been exactly one month). I've had her since I was 4 and she was a HUGE comfort when I was really sick as a child. The first day is probably going to feel like the worst day of your life. To me, it felt like I was losing my sister and best friend all at the same time. And every holiday and tradition you had with him is going to feel cold and empty for awhile. But it WILL get easier. It took me about a week to feel almost like my normal self again. A month later, I can look at pictures of her without bursting into tears every time. One thing I recommend is to pamper the shit out of him before that day comes. Take him for a walk (or a stroller ride if he can't walk) to his favorite spot. Give him some chocolate if he feels like it! It'll give you some closure that you gave him the best last days a dog could have. But probably the most important of all, truly appreciate and memorize your time petting him, the feel of his fur, and the sound of his bark. Unfortunately those memories are the fastest to fade.