It's very hard to get a diagnosis like 3 years usually or me. I can tell you that at least. I'm trying to look into a diagnosis for myself now

You can have dysautonomia (the family POTS belongs to) without having POTS! It could be that you don't have that type, like me. There are 15 types, I believe. Personally I've been diagnosed with dysautonomia, unspecified and have a similar story to you - heart monitor came back clear, orthostatic BP test didn't change enough for POTS, etc. Heat intolerance and fatigue in general are major bummers I'm well familiar with. Headaches are also common. Feel free to DM if you want any advice or have any questions, I've been dealing with this for 4 years and it sucks lol You also might have low blood pressure, iron deficiency anemia, or BPPV (benign paroxysmal postional vertigo) if you haven't already ruled that kind of stuff out. Unfortunately lots of things can cause symptoms like this and it's very frustrating for both us and our doctors

This sounds like me I feel this way for about 1 or 2 weeks then feel a bit better for a couple days then it comes back. Heat kills it! I get super light headed and my heart races… almost think I get anxiety because I know heat makes me feel worse.