Sudden Pet Loss

Hi all. A week ago today my dog of nearly ten years ran out into the street and was hit. We live on a country road and folks often speed. My mother and aunt and uncle had just returned back to the house and saw him and shouted for me. I ran down to him, hoping I could save him as there was minimal apparent damage. He could not move but saw me as I knelt down, he gave two tail wags and was gone before I could get him carried to the porch. I dug his grave that hour and will never be the same. I am being eaten alive by the guilt that I knew of the risk, but never installed a fence. We had lived at this location, with my parents were I have moved in as a caretaker, for nearly three years and there was never a close call. He would chase by the road but hadn't had any injuries, and we've never had a dog hit in the road. My parents hadn't taken concerns very seriously because of that, and I had begun to believe them as so much time had passed without issue. So many folks close to me have tried to convince me its not my fault and that it was a "freak accident" but I can't believe this, now or maybe ever. I did not keep him save and it's my fault he's gone. I am so lost and scared and lonely, I don't know what to do.