Hi "J"... Welcome to Anxiety Zone! You are by far, not alone...
I think that you will find that everyone here on the GAD forum, has at least experienced degrees of what you are going through. What you wrote, pretty much describes my history with anxiety/panic attacks/depression.
Feelings of dizziness/lightheadedness is a major component of anxiety and is actually the #1 complaint of individuals seen in an emergency room. It is by far my #1 symptom. I pretty much have some degree of it on a daily basis.
Like you, I went the whole route with physicians, and I had a cardiac catheterization too. After it was said and done, and I had a diagnosis of anxiety, I had seen six different specialists. A year or so later, I was reading an article on anxiety, and it said that the average person will see six different doctors before a diagnosis of anxiety is made. Now fancy that!
Anti-depressants are a real trial and error. What works for you, might not work for me. You mentioned starting out on 5 mg of Lexapro, and working your way up to 30 mg. Are you still on the Lexapro? How long where you on it from 10 mg, up? The reason that I'm asking this is that anti-depressants take from 3 to 12 weeks to work, once a therapeutic dose is reached. For Lexapro, that is a minimum of 10 mg. I am on Celexa, Lexapro's sister drug, and it has really helped me, but it has taken time.
GAD can last for decades. Sometimes stress factors are apparent, and other times reasons can't be pinpointed for the onset. So yes, stress can cause anxiety.
One thing that is key with the symptoms of anxiety, is to learn to accept them and not to fight them. Fighting them merely sets-up a fear-fear loop that only feeds one's anxiety and makes a bad situation worse. I know that is hard to master, but it really does help.
I hope that you find a path that works for you. The very best to you, "J"!... Chuck