First let me welcome you, but also sorry you had to come here for help. You obviously have an extensive history of symptoms.

It does appear just from reading your post that you do indeed have some significant autonomic dysfunction. However, you don't mention any specific testing. What testing have you had done and what where the results?

I'm also curious what autoimmune testing you have had since you mention the flu as a trigger. Autoimmune conditions are often triggered by a virus or bacterial infection.

I understand the frustration with the medical profession...particularly with conditions that wax & wane. You really need to have some autonomic testing though. A tilt table would confirm the POTS and orthostatic hypotension. A 24-48 hour holter and/or event recorder could pin-point the heart rate issues and determine if IST (inappropriate sinus tachycardia) is present. A 24 hours ambulatory BP could also be helpful with a journal noting times of symptoms to see if problems correlate to drops in BP...and just how far it's dropping. A sweat test might be helpful but you don't mention and temperature regulation issues.

The double vision can be associated with dysautonomia, but you should have a thorough eye exam from an ophthalmologist just to be sure nothing else is going on.

Also, a gastric empty test might prove quite helpful to determine if your GI motility is impaired.

There are various urodynamic studies that can be done, but frankly I think the patient is usually competent enough to report difficulty initiating urination and retention without specific testing. Although a post-void ultrasound can determine exactly how profound the retention is.

None of the testing is invasive, but it is time consuming, expensive, and needs to be done by someone with proper experience.

Along with testing, you should investigate known possible causes of autonomic neuropathy/dysautonomia. One should be diabetes, especially with your complaint of double vision. Have you had a glucose tolerance test or A1C?

I would also consider some basic autoimmune blood work. Have you seen a rheumatologist?

It is not always possible to determine the cause. Then you just need to focus on treatment and relieving your symptoms. You really need to know more about your heart rate & BP before you start adding things like beta blockers, Midodrine or florinef. If your heart rate is increasing from a compensatory reaction to a drop in BP, then addressing the BP might help the heart rate without medication. Some people have both NMH (neurally mediated hypotention) or orthostatic hypotension & POTS or IST. Then you may need to look at treating both. There are also supplements that some have found helpful...like B12 & benfotiamine. I have tried them but they were not helpful...BUT my dysautonomia/autonomic neuropathy is from Sjogren's damage to the autonomic & dorsal root ganglia. Once the damage to the ganglia occurs, it is typically permanent, and symptomatic treatment is the only option. The BEST treatment is to address the underlying cause...in EARLY stages. So i would encourage you to seek out second opinions and testing (if you have not had any testing).