Hi Maggy,
Please make sure that you have been tested for Lyme Disease, Babesia, Erlichia, Bartonella.
When I look at your symptoms, the migraines, heart issues, failing tilt table test, cognitive issues, fatigue..it could be a tick borne disease that has not been detected and sadly the testing can yield poor results...meaning they came back negative when they are positive.

Personally, I went years with migraines, pain, neuropathy, heart palps, etc and no one could connect the dots until I was sent to an orthopedic surgeon who did the detective work and found it was Lyme Disease and Erlichia . I lost my career to it.

If you want help, let me know.
Sorry you are suffering so and at such a young age. It is heartbreaking.
There are many nice folks here who help a great deal.
Keep plugging away with your docs....I hope you have support of family and friends who come to appts with you and advocate for you.
Welcome,
Diandra