Hey Komok, just got the feeling I need to write you a response, as I see myself a lot in your description.
I am also feeling very foggy, derealized, drunk, sleepy and many more of your described symptoms (since 4 years, I am 34) and I am also from Germany. See my details here if interested.
While investigating I also came upon the assumption it might be CCI, as the symptoms are matching and I developed cracking sounds while head movement and muscle tension in the C1/C2 region. So I paid an Upright MRT on my own, which came out negative - but to be honest I am not quite sure at that point.
Can I ask you how you got tested for CCI? Did you had an accident which caused it?
Unfortunately the situation in regards to Jugular Vein compression is very bad in Germany. Neither while having my Eagle Syndrome surgery (vein was not opening up after surgery), nor while having an Ultrasound (“Dopplersonographie”) anybody really took that part serious, although clearly seen in the CT Venogram. By the way I also struggled to get the CT payed and waiting for appointments took ages, so I paid It on my own, it was about 350 EU (just fyi).
No matter how, I also suggest getting one, as this gives a lot more insights.
Having cold showers is helpful for me, too. My explanation would be that the diameter of the blood vessels shrink, which improves the blood flow.
So as you see I don’t have any solution or good news, I just can say you’re not alone and I’ll definitely follow your story, so please keep us posted. 