It has been about 3 weeks

I lost my dog, about three weeks ago. I have lost other dogs, before, and it has always been hard, but nothing like this. I think the reason is that he was young (14 months old) and healthy and the best dog I have ever known. He was so well behaved (with one really bad exception) and gentle. I have another dog with physical disabilities and he would be so kind to him, play with him gently, cuddle with him, lick him and help him stay standing when he had trouble. He was so smart and so sweet and so good. He took off after a deer and was hit by a car. My mom says it isn't my fault, but it absolutely is. I had him off the leash, in the yard, to play frisbee. We have done that a million times, before, and he never took off. I went into the garage to get a toy for him and he took off. I called for him, but he was long gone. Instead of going after him I thought he would just come back, so I waitwed and called for him. He was probably hit not long after he took off, but I didn't go after him. I was told someone hit a dog that looked like him and I went to the spot they said and looks all over, but couldn't find him. I search all night and for the next two days. I left the gate open to our fence and slept down stairs, with the back lights on, in case he would come home. But he never did. I found him three days later, in a ditch. I don't know how long he had been dead. I don't know if I had looked in the right place if I would have found him in time (he was almost a mile from where I had looked, before). It was my responsibility to keep him safe and I failed. I hate myself and I wish I was dead. Then maybe I would be with him, again. I miss him so much!