I can really understand how you are feeling. A few years back I saw a whole family burn to death in a car. I was quite young, maybe 11, I was so chocked I didn't know what to do. There were a couple of more people there eventhough no one could do anything cause the whole thing was alight and you couldn't get near. My mum did try to get a little girl out of the car but then she caught fire too. It was really horrid. My mum was fine by the way but all of the family died except the two parents. They had to watch their 5 children burn to dust.
It's something that fades with time. I don't think of it very often nowadays but it's still there. I do think you should talk to someone profesional though. That can really help, if only to help you sort out your attitude to what you have seen and how you relate to the event. It helps if you get it explained to you that there is nothing you could have done. It's hard to accept by yourself, i think so at least.
I'm very sorry you had to watch that.