If you have already done full cardiovascular, neurological and vestibular workups, you might find a therapist who specializes in anxiety and get a workup. The things that you name as easing your symptoms are also the classics for easing anxiety, which suggests (doesn't prove) that your symptoms are coming from anxiety.
Many of your symptoms: lightheadedness, brain problems (inability to put thoughts together or concentrate on tasks) are consistent with hypo or hyperthyroidism (too little or too much thyroid hormone). They are also consistent with anemia, too little iron in your blood. If you haven't looked at hypothyroidism symptoms yet, I suggest doing so -- muscle control, cognitive impairment, inability to focus - can all be thyroid related. (I'm not saying it is in your case, but since you are trying to eliminate possible causes that would certainly be one to look at.)
Do you experience afternoon fatigue? For me, this was one of the worst hypothyroidism symptoms that I just could not kick until I got to the right hormone level. Do your hands and feet get cold throughout the day? Is your resting heart rate low? Mine would sometimes read in the low 40s when I was very hypo - I am an endurance athlete, but I'm not that good of an endurance athlete for that to be normal for me. When I was hypo, it would average around 50-55ish. Now that I'm no longer hypo, it is around 58-60 bpm resting, most days. It is insanely high in doctors' offices though, because I get nervous and throw off the blood pressure and heart rate tests, every time. Do you have something that can keep track of your heart rate throughout the day? I've noticed when I was still experiencing the extreme afternoon fatigue that my heart rate would drop to its lowest point all day right around 2-3 pm every day.
You say that your blood count is not abnormal. Have they specifically checked for ferritin or hemoglobin? (I say this, because I am someone who had both hypothyroidism and anemia at the same time and both can cause similar brain fog/forgetfulness/concentration problems because both of them cause less oxygen to get to the brain.)
Alternatively, this could be a circulatory problem - if blood is not getting to the periphery as well as it should. that can also be caused by a heart problem, circulatory system problems, or lung issues. I assume your doctors have done an EKG to check for heart problems, and at your age, hopefully this would not be the cause. If there is a valve problem or an arrhythmia, it could cause less oxygen to get to your brain.
Since this started 2 years ago, I doubt it would be caused by the current pandemic, but any lung damage that makes it harder to get oxygen in the blood can also cause similar problems.
I'm not a medical expert, but I wanted to share with you that your feeling better after exercise is exactly how I respond to exercise. I no longer have a thyroid and am fully dependent on supplemental thyroid hormone, which I take in the morning (T4, the inactive thyroid hormone, and T3, the active form). I also take some more T3 at around noon. On days I run 7 miles or more (about 75-80 minutes), I have much less afternoon fatigue than days where I run 5 miles or less (less than an hour). I'm not an energetic person, but I feel fantastic after running, and on days I don't run at all, I get extreme afternoon fatigue, with the same dose of T4 and T3 I always take. I think that either running/exercise helps me convert T4 to the active T3 better, or the increased blood circulation helps keep my energy levels up throughout the day because more oxygen is getting to my brain since my heart is still working a little harder after I'm done running (or could be both? I have no scientific evidence any of this is true, it is just how I feel). Either way, it sounds like exercise helps improve circulation at least briefly, for both of us.
I am so sorry you have to deal with these issues! I've struggled with hypothyroidism for the last few years, and after two thyroid surgeries, two years of trying to get to the right dose of thyroid hormones, and realizing I was also anemic and needed two more surgeries (for a fibroid), I'm now finally at a good thyroid hormone level and I feel like I have my brain back. The brain fog/forgetfulness/inability to focus sucks! Sometimes it feels like I was a completely different person when I was going through it all.
Hopefully Barb135 will chime in as well, because I think she is much more knowledgeable than me about other possible causes. I can't tell based on the symptoms that you have hypothyroidism or anemia, and I'm sure there are other problems that have not been tested for that can cause similar symptoms (I just don't know what all of them are).
I'm hoping you find a solution and figure out what is wrong. I know when you are dealing with these types of symptoms it can feel overwhelming and like the whole world is passing you by, at least that is how I felt. Many friends and loved ones may have no idea what you are going through. It was so difficult to explain to people what was going on, especially when I wasn't even sure and I was told I was at a normal thyroid hormone level and still experiencing symptoms. Once I figured out what was wrong and finally got it fixed, my symptoms went away. I hope yours will too, keep looking, keep bugging your doctors, keep asking for tests. I had to beg my ENT to test for Hashimoto's (which can cause hypothyroidism), even though my thyroid hormone levels were "normal", and I did have Hashimoto's (and a thyroid that needed to be removed, unfortunately).
You are young, and when this gets diagnosed correctly and you are feeling better, there will still be opportunities for you. I can't say how long it will take or what the actual cause is, but for me, I thought I would have to deal with extreme fatigue, brain fog, inability to concentrate, forgetfulness, and lack of motivation for the rest of my life, but thankfully, this isn't the case for me. Now that I'm no longer hypo, I've spent some time grieving for all the time and opportunities I've lost, but also realizing that what I went through has made me a more sympathetic person, and stronger in general, and now I'm looking forward to possibly returning to a career pathway I left several years ago, when I was hypo but well before I knew what was going on.
Most, if not all of your symptoms sounds like Dysautonomia (possibly POTS). I recommend checking out Dysautonomia International's webpage to view the symptoms and see if it's a good fit. My Dysautonomia was diagnosed by my cardiologist and later confirmed by a neurologist, but finding doctors knowledgeable about Dysautonomia is challenging. Good luck!