It is good that you are realizing there is a connection to what happened and something in your past and have talked about it and written it out too. Losing control of one's car and having an accident is definitely scary, but then having a police officer/authority figure talk down to you in an angry tone can make it worse, more traumatic. It's not surprising if one has a history of trauma that involved someone older with authority be mean that could come forward in a flashback be it emotional or visual. Unfortunately, police officers can be mean and quick to blame. I believe you when you say you were not speeding and it was raining, I had an accident like that myself so I know the rain can be slippery.

It's important that you acknowledge whatever comes forward and remind yourself that you are ok now. It's good that you got to talk this out with your therapist and even here where others can listen and comfort you. One of the things I have realized is how in a lot of my past traumas I did not have a witness or someone there to comfort me. We cannot change our past, but one thing that helps tremendously is to have access to someone that will listen and validate and comfort and in a way that we finally feel "safe".

Unfortunately, when it comes to police officers they tend to get calls where people have been reckless and careless and after a while of being exposed to that constantly, they can get mean and short tempered the way you described. I had a student who's father was a police officer and he told me that in his job it's a steady diet of dealing with people that are mean and angry so they definitely spend most of their time seeing and dealing with the negatives in humanity.