Many individuals that suffer with generalized anxiety disorder (GAD), panic attacks, and depression, have no apparent trigger that is causing their symptoms. Like you, I fall into the category. Sure, I have stressful situations that occur in my life, who doesn't, but I feel I don't worry or ruminate over them. Consciously, that may be true, but our subconscious maybe processing these stressors, and forming triggers. It's outlet is that of anxiety/panic attacks/depression.
You mentioned dizziness/lightheadedness. This symptom is the #1 complaint of those individuals being seen in an emergency room. Dizziness/lightheadedness is a main symptom of anxiety, and it is also my #1 symptom.
It's great Dan, that you have seen your regular physician, had tests done, and have ruled-out many disease processes. I did the same thing some four years ago. I even went as far as to have a cardiac catheterization.
These days I have a psychiatrist that is in charge of my mental health needs. She has provided me with a diagnosis, treatment plan, and has me on an anti-depressant. If you haven't already, I would suggest you consider seeing one.
The very best to you, Dan!... Chuck