I am concerned about the combination of easy exhaustion & a resting heart rate of 90?
The normal heart range range is 60-80 in a healthy hearted person at just about any age. A slight overweight issue may increase this, but 10% + over seems high to ignore ALL heart issues on "I won't do any more tests" kind of attitude. Which I would also suggest may be a problem, none of us needs a Doc with an "attitude". We need Docs who won't quit until they find the problem, which sometimes means looking in the same closet for awhile.
You aren't new to the treadmill, (which I use 3-5 times a week). I have never felt so tired from a workout that I had to nap and could not keep my eyes open. I believe you have some medical issues that are not diagnosed.
My best suggestion for the moment, is to find a new General Practice physician, make a list of ALL symptoms, and then see the new Doc to see if a new perspective can help. As you said, your first Doc completely whiffed on the Anxiety diagnosis. Docs are people, they miss things and make mistakes like anybody else. Last time I heard, it was still called the "practice of medicine", because nobody has it perfect.
Hope this helps and Good Luck in the search