I don't know your situation, but something very similar happened to me when I was under a lot of stress, away from my family and after several relationships failures. My confidence level was so low and I think that was triggering the whole situation. I didn't trust myself anymore. I didn't trust I was doing simple things correctly. It's an extension to the problem "Did I really lock the door?", but with much deeper and frightening questions such as "Did I really avoid that cyclist or did I hit him without noticing?". Damn, I should really go back to the "crime scene" and make sure everything is fine! Yes, I did that and when I rationalized it I knew it was ridiculous, but the feeling of doubt and stress was so strong I somehow had to go back and check it out. I would look at my car for "proofs" of innocence like you. It could help a little bit, but there was always a new scratch somewhere that could trigger the doubts again. Back to square one. The thing is, you do not want to train your mind to continue with this line of thoughts. Finding "proof" or going back to the "scene" only adds confirmation that there is potentially a real situation when we both know there isn't. It stimulates the brain, triggers other questions, etc. You want to break the pattern as soon as possible. The more you encourage your mind, the more questions, the more stress, the more doubts you will feel. I will give you some tricks that helped me tremendously.
As soon as the symptoms appear, recognize them. Know what they are. Not feelings of something real, but something your mind is creating.
As soon as doubts appear in your mind, try to dismiss them (I know this is hard). Fight them. Never encourage them by looking for proof or going back. What helped me: running a 5k asap or calling a friend, but more importantly stop caring. I said to myself: You know what, you did your best. You drove the best you could and if this "accident" really happened so be it. I didn't mean to and I'll pay for the consequences. I know it didn't happen, but if it really did so be it. I'll deal with whatever punishment. etc. You have to train your brain not to worry. Not to care, because it's not real anyways (you know it rationally, you have to use that to your advantage and train your brain).
Also, always remember that if what you think happened really happened you would know. Yes, in addition to feeling guilt and stress, you would know exactly how it happened. There would be memories. Sounds. Images. Odors. Understand that it's totally impossible to kill someone without realizing it. Impossible.
The sooner you stop encouraging your brain, the sooner the symptoms will disappear. For me, at first it took a few days. Then a few hours. And know not even a second. It's not instantaneous, but once you know the symptoms and act on them they will disappear eventually. And at some point they will disappear instantly.
Don't give up!