Make a poster that says "My rationale is that I shouldn't be worried about anything serious because I doubt it would have worried me if I didn't have a history of TBI. Also, I doubt he hit me really hard out of the blue, and perhaps the reason that it registered as being harsh was because of the surprise element. "

Then, anytime you start looking for symptoms, read the poster and realize you are smart enough to put your anxieties in a rational light.

You have already outsmarted the anxiety. Do it again and again.

And, tell your friend that the next time he wants to be a irresponsible jerk, stop and think about the ways somebody can pay him back.