How can I get this off my mind?

I was driving today and i rear ended someone. I didn’t even realize they hadn’t moved for the green light because I was looking at the light instead. I don’t know why I wasn’t paying more attention but the next boom I heard alerted me and I realized I had hit the poor car in front of me. I was completely astonished that it just happened and everything ended okay with little to no damage on either car. Still as I sit in bed I cannot keep my mind from reliving the moment and thinking, how could I not have been paying attention? I now can’t sleep and every time close my eyes I still feel the jolt of our cars hitting and the shock that went through me. How can I get my mind off of it and fall asleep?