Some of the comments here bother me. First, it is your legal duty to act when you witness an emergency. Not only that, but if you're not going to try and help someone when you have the proper skills to do so, why are you in this field. Even if the chance is so low for survival on that pt, anything and everything should be done to help so long as the scene is safe and you're pt is decapitated. I am not talking specifically about this case, but in general. What if the pt has a life threat that you could change or at least help until better care arrives and you just sit in your car or worse drive away?
Also, just because a pt is a blunt trauma arrest you're not going to even attempt to work him? Medics may have their reasoning, so this isn't directed towards them, but rather basics. I was first on scene, off duty, from a 3 story fall. Agonal resps, no pulse, but I started working him anyways. Medics arrived, did their job and the pt lives to this day to tell his story. It may be that I am new and haven't been on the job enough to lose hope, but even if 99% don't survive, how do you know that your pt isn't the 1% that does?
To the OP, the best thing I can say is my old instructors motto. "We didn't create the problem, we're just here to help." Like many have said, its a learning experience so that next time you will know what to do. 180 hours is not enough to give us any sort of muscle memory, and nowhere near enough to teach you how to stay calm and focused in what seems to be one of your first true emergencies. As someone else stated earlier, 1 call in real life is equal to 10 in class. Don't beat yourself up, you at least tried and that is all that matters.
I know I am new to this forum, and with only a little under 200 calls under my belt new as a basic as well. This is strictly my opinion and not a personal attack on anyone. I wrote this strictly so others can see a different light on the situation, and maybe to inspire those who said they wouldn't respond to think for another second the next time they're caught in a similar situation. I would hope that if I was hit by a motorcycle and had even the slightest chance for survival that one of you would work me through the ground as I would do the same for any of you.