Sometimes it's difficult to get a diagnosis, and sometimes you end up with crap doctors who run more or less the same tests for every patient and write off anyone that doesn't match as a hypochondriac/lunatic/time-waster. And sometimes doctors literally know less about your condition than you do yourself.
If you've never been in this position yourself, I can understand why you'd be sceptical of the OP's claims, and indeed I'm sure there are lots of people that are convinced they have a particular condition despite what tests say.
For context, I was written off by my NHS neurologist because I didn't match his standard battery of tests. My circumstances and symptoms pointed strongly to a small fiber neuropathy, but he scoffed and said it was very unlikely, and refused to do a skin biopsy to confirm it. Instead he labelled me with fibromyalgia, and hoped I'd disappear - but when questioned it was abundantly clear he literally knew next to nothing about fibromyalgia, and presumably used it as a generic label for "time wasters". Eventually I did get him to do the biopsy - which they fucked up. When the results were negative, he again tried to get rid of me, and I had to fight to get him to redo it properly - lo and behold, it was positive.
I had a very similar experience with endocrinology - I have a rare endocrine condition, but the symptoms point absolutely towards a specific diagnosis - instead, I was basically told to "stop this silly nonsense", and it wasn't until several years later that I was diagnosed. NHS endocrinologists have a terrible reputation for anything that isn't diabetes.
People rave about how wonderful the NHS is, but my (unfortunately substantial) experience has been shambolic. It's my observation that doctors (and consultants and registrars alike) don't update any of their knowledge or training since they left medical school, even if that was 40 years previous; they have no interest in recent medical developments; they have no interest in writing case studies or undertaking research; they could basically be replaced by a flow chart.
/rant