It has to be frustrating to not find someone who shares your story on here. What you went through sounds awful! I'm so sorry that happened.
As far as how you handled the situation at the time, you did the right thing! It sounds like the victim knew he was going to die, and wanted you there for comfort, which is what you gave him. While there was nothing you could do to save his life, and for that reason you felt powerless, I don't think you can put a price tag on emotional support. You gave him "comfort care" to use the words of hospice programs. If you had left, he probably would have died before the medical crew arrived on the scene anyway. There is a lot of research to back up "staying with the victim." It's not like you could have pointed to the probably immobile other people in the car and said, "You go call 911!" So you did the right thing by providing him with a good quality of life for his last remaining moments on Earth.
You DO fit in here. A trauma is a trauma. PTSD is PTSD. When I watch programs of veterans talking about their PTSD, I don't relate to the specific events, but you can bet that I can relate to how they are coping/not coping.
Here is a good definition of trauma as described by a treatment program I attended:
"Chances are that whoever you are, you or someone you know has been affected by trauma. To put it simply, the scary, painful and yucky stuff that happens is what trauma is made of. Often, people believe that trauma always pertains to war. Though war is definitely traumatic, there are other events that can have the same effect on us. Verbal, emotional, physical or sexual abuse, being the victim of or witnessing a violent crime, responding to a horrific emergency call, natural disasters, and car accidents are some examples of traumatic events. We all know the stories. Those of people we know-- they may be family, friends, neighbors, coworkers, clients, movie stars, musicians, athletes, and people on the evening news. Some of us know the stories all too well, because those stories are our own."
-Link Removed