POTS and MCAS (often co-occur) were the first things that came to mind looking through the symptoms. POTS is a well-established, measurable thing, MCAS not so much - the NHS doesn't even formally recognise it.
I'm pretty sure I have MCAS as I'm plagued with allergies (I reacted to almost everything on the list apart from cats (hooray) and dust mines (initially the prime suspect), rashes, boiling hot and sweating horribly within a minute of walking around and shivering within a couple of minutes of sitting down, horrific night sweats since I was 22 (wringing out PJs level at times), dry and itchy eyes, dry lips, chronic reflux, skin tears at the slightest thing, covered in bruises with no idea where they come from, random 'internal' itch episodes - like my blood is on fire - in my hands and feet that have been so severe at times that I've been jumping up and down (was sent to secondary care for that, nothing came back in tests so they just shrugged and gave me stronger antihistamines) etc etc.
I've seen the GP about individual things over the years but things tend not to be looked at as a whole. They were good with testing when I queried if there could be something autoimmune going on but ANA was negative and no RF either.
A HCP commented on my high HR recently. I said it's always been like that and presume I have POTS. Unbeknown to me she spoke to my GP about it who asked me to book an appointment, so I'm being referred to cardiology for a tilt table test which was a bit of a surprise. I mentioned the possibility of MCAS but he said let's investigate the possibility of POTS first.
You're not crazy. It can take years and years for many conditions to be diagnosed. And even if you never get a clear explanation for why you feel the way you do, it doesn't mean it's in your head.