Tyler, You would need to cause very serious damage to your car to have any stopping force that could cause a concussion.

Your car's headrest absorbs a lot of impact and reduces the G forces substantially with that energy absorption.

And, NO. You were not going 10 to 15 mph. More like 3 to 5 miles per hour. 15 mph backwards is an exciting ride. Even 10 mph is.

A mild neck strain could have happened.

You say you don't care what anybody says about anxiety. Then, you say your anxiety is skyrocketing. Your anxiety is far more impactful on your cognitive skills than any mild concussion. The neurochemicals caused by anxiety will have a direct impact on your memory and cognitive abilities.

I struggled in school. My struggles were not caused by may many concussions. They were caused by how I stressed during test taking. I would know the subject matter with mastery. The instructor knew I knew the subject matter. During tests, the increased stress would cause a mental block that impacted memory recall. As soon as I handed in the test/exam, I could recite the questions and remember and give the correct answers.

SO, No. It was not a cognitive issue. It was a 'dealing with stress' issue.

NO. Not a concussion. There was not even close to enough impact force. You already know why you feel so bad.

You need to consider how you can best treat your struggles. Concussions are not the cause of your struggles.