I'm so sorry... I have a dog with Addison's Disease and epilepsy. He takes 17 pills a day and a shot every 25 days, and I am constantly faced with the possibility that at any moment, he could go into grand mal cluster seizures. (This usually tends to happen on weekends when the regular vet isn't open, of course, and so I have to take him to the ER.) It's terrifying, every single time. So I've had to face the possibility numerous times that we might not get him to the vet in time, or that this might be the time that the valium to bring him out of it doesn't work.
I sobbed and sobbed when my cat died, and I am WAY more attached to my dog, so I can't begin to imagine how I will feel when it's his time. Dogs are so very much like children who never grow up.
You are doing the right thing, though, in not letting him suffer. Just remember, dogs have no concept of longevity. Your puppy doesn't think he's dying too soon, because he doesn't know even understand that life is finite. For him, it'll just be like laying down to sleep. But he will sense your distress, so the best thing you can do for him is try to make the time he has left as much of a party as possible, and to let him know you love him, and are proud of him as he goes.
Will keep you in my thoughts...