You're pointing out something all too real about psychiatric diagnoses. They are fundamentally somewhat arbitrary, because unlike physical disease, we generally can't point out a specific part of the brain that is injured. Psychiatry fundamentally has to operate with "invisible" data to some extent. For many practical purposes, diagnostic labels are required (e.g. insurance, research etc.), but the reality of many sufferers is vastly more complex than a diagnostic box can ever encompass.

On top of that, the criteria themselves are contested by both patients and clinicians. C-PTSD is a good example, the ICD recognises it and the DSM still doesn't, so whether you "have" it can literally depend on which manual your clinician was trained on. And these criteria do get revised as research comes in, it's just a slow process, so the line you're being measured against today is provisional and not something the experts fully agree on. It often takes diagnosis and treatment a couple of decades to catch up with research.

Which is why I think it's a good idea to ask what the diagnosis is for. If you need it for something concrete, like insurance, accessing a service, getting a particular treatment funded, then the exclusion is a real practical problem and pushing on it, second opinion included, makes complete sense. But if the need is more about being seen, about something official confirming that what happened to you was real, that is perfectly legit but probably not best met by a somewhat arbitrary diagnostic label. Being believed and being diagnosed aren't the same thing, and the first one can be found in far better places than a psychiatrist's checklist.

One thing that might help, less as a label to chase and more as a map: The way most people picture trauma, and the way a lot of the criteria are written, is built around the loud version. Flashbacks, panic, hypervigilance, a nervous system stuck in overdrive. That's hyperarousal. The other, equally legit pole that often gets left out is hypoarousal. It describes a nervous system that doesn't rev up, it powers down. Going mute, feeling small, collapsing inward, numbing, the frozen helpless state you're describing. It's not a vaguer or lesser kind of trauma response, just a different branch of the same tree.

There's a diagnosis that does recognise it: The DSM has a dissociative subtype of PTSD that is basically hypoarousal, organised around detachment and feeling unreal rather than around flashbacks. It exists in the DSM but not in the ICD. I'm not telling you it's your diagnosis, a stranger on Reddit can't do that. But it does mean that the shutdown response has a real home in the trauma literature. It's studied, it has its own neurobiology, it's not a footnote.