Here is what I finally did that worked: I bought an exercise watch with a heart rate monitor that I could set to alarm when my heart rate got too high (for me, this is anywhere over 135bpm). I have found that if I exceed that threshold, all my symptoms come back. I also use the monitor to limit my tendency to be stubborn and just try to push through the fatigue, as this is a strategy that never worked. If the watch alarms, I slow down, no excuses. I have also found that if I take 20 mg propranolol at least an hour before I start exercising, my heart rate will stay lower during exercise and I am able to do more without getting as exhausted afterwards. The same thing is true of drinking water. I make sure to have at least a glass or two of water in the 30 minutes before I start exercising.
The key is to take things slow. And by that I mean exercise so little that most people will probably laugh when they hear about it. I started on the stationary recumbent bike for 2 minutes at a time (starting with exercise that does not involve standing makes things a lot easier). I did this for a week or so then added another 15 seconds. I repeated this process until after about a year I was able to ride the bike for 15 minutes at a time. This was a huge accomplishment for me and it has definitely improved how I feel. In fact, just recently I have starting jogging on the elliptical using the same strategy of starting with a couple minutes and adding 15 seconds every few days . I can now jog for 15 minutes at a time. If you had told me that a year and a half ago, I never would have believed it.