I realise most of the "events" after the event that I define as "THE concussion" are exactly as you describe - anxiety cycles and thought loops. The ones that I can instantly rationalise and dismiss as non-events last a few seconds. The ones I struggle to defeat with logic drag on for 1-2 weeks and in some cases, a few months - these longer ones tend to come in waves. Stress tends to keep them alive for longer.

It seems my mind and body calibrate the symptoms I am going to have based on my appraisal of the scenario severity: type of impact mechanism, momentum, point of impact on head, height or distance of motion, hardness of impact and rigidity of object.

The other week, I was sat on a "build it yourself chair" I bought from Amazon and the thing broke while I was sat on it, wearing noise-cancelling headphones (so a bit detached from that instant proprioceptive reflex). I remember the fall backwards was partly controlled, due to the chair slowly folding under me and I sort of slid down the chair before it hit the ground.

I definitely remember the impact sensation in my upper back as the chair hit the floor - not my head.

However, within 30 seconds or less, the anxiety and doubt kicked in. I could not trust myself to accurately remember the event and started gaslighting myself with thoughts like "you did hit your head, you're just in denial / or you were knocked out for a millisecond and have forgotten the impact". Then I try to rationalise with "ok, if I hit my head, why no bruising, soreness, grogginess?"

Logically, my mind can see the evidence, however, my brain chooses to believe the feelings/fear/uncertainty. A previous therapist identified I suffer with Intolerance of Uncertainty, where I focus on making the the worst-case scenario a reality as a way of avoiding living with the uncertainty.

For me, Anxiety + Uncertainty = "But, what if...?", unfortunately, But what if + Rumination + More Anxiety = "I am certain the 'what if' has definitely happened".

Logically, trusting screwed-up maths formula produced from the emotional brain makes no sense, yet still I believe screwed-up maths formula produced from the emotional brain.

Even though I know all the above, I still don't trust myself to be ok.

I'm having another go at therapy for a bunch of stuff and it's ironic the chair collapsed 10 minutes before the first session with the new therapist.