Hi,
I hope that the professional help will ease some of the burden. I witnessed a C172 crash in Sydney a few years ago, albeit from a km or so distance (at the airfield). It was very challenging especially because, despite the evidence of my own eyes and ears, I had a hard time convincing anyone that the aircraft was in serious trouble. Eventually - maybe only minutes, in retrospect - one of the locals (not knowing me from bar of soap) could see the depth of my concerns and organized a search circuit which, after a bit of looking, found the aircraft about the same time as the other ground reports started coming through.
I didn't make it to the crash scene, since at the time I was recovering from leg surgery. One of the police later told me to be grateful for that impediment although I took a small amount of comfort from the fact that the 4 occupants apparently died instantly. What bothered me immensely in the subsequent weeks was the difficulty I had in raising the initial alarm but one of the ATSB guys later told me that it's a very common thing with humans to start with the denial response.
I had no specific psychological symptoms, perhaps because I didn't see the aftermath, but people tell me I was grumpy for about 6 months, maybe (I think) wondering about how the world just goes on in the face of such a tragedy. Strangely, I didn't relate what I saw on the day to my own flying, since I was able to tell myself, with some justification I think, that I would never have gotten myself into situation of that other pilot. In the end, it was the joy of flying that helped me get back to a more normal perspective over time.
Occasionally, I do still see in my mind the 172 wing drop, the powered dive, and recall the terrible 'whump' sound. I remember that it all seemed to happen in the interval between two heartbeats. It's also a great continuing comfort to me that the occupants did not suffer. As a practical outcome, I am much less tolerant of people who don't want to hear bad news - in all aspects of my life, but especially aviation.
Once again, good luck.