OP, I was in your shoes 7 months ago. Your post has me nearly in tears because it's pretty much what I wrote back then. I was involved in a head-on collision where I saw it all happen and yet could do nothing to avoid or prevent it. Other driver was in la-la land and came around a bend on the wrong side of the road. At the time, I was incredibly calm while waiting for the emergency services to arrive, probably due to having a lot of first aid training myself so I knew what was coming. That calm (or complete shock!) lasted until I got in my front door after spending the day at A&E and I crumpled. I knew at that point I was in serious trouble. I've never suffered from stress or anxiety prior to that accident but within a few weeks I knew I needed help as I wasn't sleeping - pain and nightmares/flashback related - was not in control of my emotions at all, was spiralling into a dark place I'd never experienced before (not the extreme dark place but dark enough for me), couldn't see how I was going to move forward with recovery as I'm usually so active and fit, was closing myself off from my friends etc etc. Suffered insomnia for about 4 months and the 6months post trauma were probably the most difficult of my life. Usually a confident driver, when I eventually got back behind the wheel, I was a mess. The amount of times I had to stop and pull over because every car was out to get me (obviously they weren't!). I was diagnosed with PTSD and have gone to someone over the past number of months. Now, I'm doing a lot better but to be honest, there isn't a day goes by that I don't think about that day as I'm still having chronic pain that reminds me of it. In the gym, if someone drops the heavy weights, my heart rate soars and I jump a mile. Brings me back straight away. The thing about it is, it made me question everything, I nearly died in that accident and in a smaller car, I'm in no doubt that I would have but luckily I drive a strong one and it protected me to the point that in the bigger scheme of things I did come out of it quite lightly although I have ongoing problems.
How have I coped? By being kind to myself. I'm a lot more willing to walk away from things that will stress me out. I'm as frustrated as hell and can still land on the floor bawling my eyes out, even being in the gym doing my rehab strengthening exercises will have me in tears because it reminds me of why I'm there.Being in constant pain doesn't help either. I just take every day as it comes and try and muddle through as best I can. I'm not too sure the Psychologist I've been going to was enough of a trauma specialist to be of max use to me though, she kept wanting to delve into my past which was nothing to do with why I was there at all!! So just make sure you're happy and comfortable with who you go to. This accident absolutely floored me in all kinds of ways and not something I'm used to at all. The other driver is due in court in the next few weeks for prosecution and I know I won't sleep that week.
Just be good to yourself, get the help you need, don't be afraid to ask for it. And don't be afraid to talk. Use your doctor if you have a good relationship with him and don't try doing it all alone. Also, healing takes place at your pace and on your terms, don't let friends try telling you that,"sure you must be over it now..." . You're over it when you're over it and to be honest, even though we tell ourselves, "weren't we lucky to walk away from it?", nobody really just "walks away" from a traumatic experience like that. I don't know how this forum works as I just saw your post and saw it was recent, but if there's a pm or email option on it, feel free to do so if you're struggling.