My experience is quite similar, except my issue was recurring severe unexplained pain with scarring of the joints. I've come to believe the same conclusion that chronic fight or flight is the biggest factor and have healed myself within a year.

The difference is that I've come at it from a different approach, where this fight or flight is a neural pathway or habit of the unconscious mind that you can train yourself out of. Although trauma can probably be a significant factor on how people get to this state, "the way out" (title of the book by Alan Gordon that explained these ideas) is probable the same and I believe not all people with these issues have significant trauma, although in some it may be necessary to address as an underlying cause.

As for how you can train yourself, it's explained in the book mentioned above as well as the free podcast "Tell me about your pain". Since the podcast is free so the techniques are public knowledge, I believe I'm allowed to summarize my take:

1. Constant "problem solving" and overthinking about your health can put you in a state of stress that heightens your perception of your own body which in turn convinces you that something is seriously wrong (eg: you may hear your own heart beating very loudly even though it's beating normally, or you may feel pain even though the nerves are transmitting normal signals to your brain). If you've been to many doctors and they can't find an explanation, there's a chance they all missed a serious problem, but there's probably a bigger chance that there's nothing physically wrong with your body but your nervous system is causing the altered perception and over reaction. It can be freeing to realize you don't need to try a new diet, or spend all your free time on pubmed, or go see an expensive doctor that prescribes expensive supplements. If you have a "neuroplastic" issue these things will not only not help, but actually hinder your recovery.

2. Catching the "what if" thoughts. You can't stop yourself from going to a worst case scenario, but you can stop yourself from buying into it. Every time you think something like "what if I actually have an injury that the MRI and X-Ray missed"? You can reply to yourself how unlikely that is. The nervous system is part of the subconscious so making yourself feel calm will probably, over time, calm it down.

3. Somatic tracking, which is the action of paying attention to a painful, strange or otherwise uncomfortable sensation in your body through a lens of safety. Eg: wow, it's incredible that my brain can usually filter the sound of my heartbeat but now I'm able to hear it at full volume. If you understand that the sensation is safe and you have nothing to fear, paying attention to that sensation will slowly make it go away over time (not necessarily in one session, which although it can happen is not the goal). Your nervous system highlights pain or sensations because it believes they're dangerous and wants you to feel them to avoid further injury. When you pay attention to them in a lens of safety it understands that there's no danger so they get deactivated.

4.Paying attention to things that feel good and being mindful in the moment about them. Whether it's feeling your breath in mindfulness meditation or taking a walk in the park and consciously being present and enjoying the experience.

5. Filtering what you consume. Media and content nowadays tries to appeal to our most basic instincts, especially fear. Reading news can put you in a state of alert. Watching shorts or tiktok can overstimulate you. Try to be comfortable with stillness and doing nothing. You don't need to fill every free moment of time with your phone.

6. Have you ever though: if I was better I would do X, or when I get better I'll do Y? Being sick can bring apathy that can stop you from doing things you enjoy even after being physically capable. Actually doing them and learning to enjoy them again isn't just a possibility, it's a necessary part of recovery.

I'm sure there's more, but this is what worked for me to go from bed-bound with full body pain (with 3 surgeries to remove internal joint scarring without any injury) to being 98% recovered in less than one year. If any of this resonates with someone reading this I'd recommend listening to the podcast, it's free and got me 70% of the way there even without the book.