It's ok to feel bad about it and it is good to grieve sometimes. Trauma does affect everybody in different ways and there is no easy way to stop being affected.
You can always look at personal problems like a dog problem. People need exercise, discipline and affection as much as dogs do. So go get a new hobby and work at it and enjoy accomplishing something. Go learn Tai chi. That stuff is good for the body and the mind.
Your dog was just unlucky if he got hit by a truck.
I've watched my dogs run across busy streets after cats and am surprised they didn't get hit. It is just luck. Dogs don't realize roads are dangerous. They are driven by instinct. We cannot predict or control everything and what is the point to try.
Hope it gets better for you anyways.