Same, that one is completely unrelatable to me. My boyfriend can do it, but we suspect it's actually dissociation, a trauma response.

Ding ding ding! For me, the thoughts become so pervasive/overwhelming that I either retreat completely, or get lost in the sauce. It's not uncommon for my spouse to check in on me while I'm on the toilet. "You've been in there for an hour." Zero recollection of time passing, and little to no memory of thoughts. Or I'll be in the shower, have a stray thought snag a neuron, and who knows how long later I'm still standing there holding the soap, unsure of what I've washed already, but shaken because I just mentally experienced a dozen different versions of a traumatic event that hasn't actually happened. The former is as close as I get to head empty, the latter is everything firing at once in the least productive way possible.

I've tried to so many times "clear my mind of thought" as people say to do for meditation, but all the attempts have ever done was leave me more stressed than I was before. My brain does not shut the fuck up. Ever. I've been suffering from insomnia as far back as I can recall, all because no matter how tired I am, sometimes my brain just will. not. be. quiet. Everything is a potential stimulus. Any minor sound, the feeling of my bedsheet, even having my partner turn over could remind me of some obscure memory or story or fact, and my brain doesn't stop, it just changes direction.

Loud and clear. For meditation, I've found that guided works better for me. And instead of "clearing my head" (ha), concentrating on abstract visualizations related to the guidance helps. For sleep, I have to break all of the "rules." Have something to concentrate on like a game, video, or book until I feel like I'm "ready." (Like dropping the phone or controller.) And then have music playing low to take my attention as I fall asleep. Without it, as you said, any little stimulus is enough to send things into overdrive and undo any sleepiness. Doubly so if the stimulus leads to anxiety.

Also, fuck doctors that won't listen. And double-fuck those that insist on trying to cram everything into boxes that they're familiar with, to the exclusion of maybe just MAYBE the person living through the experience has a better read on said experience than you do.