I just started taking Toprol XL 50 mg about a week ago. I feel a...

Personally, I've read about alternative methods of dealing with POTS. Because it's caused by low blood volume in the legs (Am i correct?), I've heard that people with this can benefit from wearing compression stockings (very appealing right) as well as adding magnesium, calcium, and more fluids and salt to their diet. However, even on these websites, they have all said that blood pressure medication (such as toprol) is sometimes necessary for this condition. You have a really large change in heart rate, so maybe that's why the doc felt it necessary to put you on it. I'm on it too, but for different reasons, and I don't like it b/c I feel like my IQ has dropped about 20 pts since beginning it (super brain fog).
If you have a pulse in the 40's from being so highly conditioned as an athlete, as long as you never have had symptoms from it, lowering it a little more should be ok. Again this is just what I've read about this topic, i'm no expert. Maybe look into cutting your dose with your doctor, to alleviate some of the symptoms.