Claired, You may likely have an upper cervical / brain stem issue. POTS can be an autonomic nervous system dysfunction. That is often a brain stem problem. No amount of vestibular rehab will likely help if your autonomic system needs healing. There are brain stem mini-strokes that can cause these issue.

A Physiatrist (Physical Medicine and Rehabilitation) often associated with a Neuro Rehab hospital may be able to help. As Jo*Mar said, an Upper Cervical Chiro may be helpful, too. The Vitamins sticky at the top has links to various Upper Cervical Chiro organizations for a referral to your area.

Have you had an MRI or MRI/MRA or MRI with contrast to see if you have proper blood flow to your brain stem ?

The myriad of ways POTS can manifest and be caused makes your case difficult. It appears you say that the POTS preceded your concussions. Is that correct ?

btw, What area are you in ? Maybe we can help you find a health care provider than can help.