I have an extreme reaction to extreme aerobic exercise. I can't tell whether the magnesium makes any difference or not, I just think it's good on general principle.
magnesium basically lowers your blood pressure. That's a huge oversimplification, but that's the idea. If you are already getting magnesium in your diet, you could actually be making your situation worse by supplementing with it. I love raw almonds, they are a staple of my diet. They are very high in magnesium. I also have low blood pressure naturally. When I added magnesium to help with anxiety, I got pretty exhausted about a month later, and then started having fainting spells.
I'm not saying magnesium is the culprit. But when you supplement, you do need to factor in what you are already getting into your diet and where you are actually deficient. You might try cutting out the magnesium supplement and see if your endurance improves.
Also, if this is helpful: GABA has some clinical to back it up, but it also is known that it doesn't cross the blood-brain barrier. So when you take it, it's working through your metabolism in a way that has been proven helpful for anxiety in a few studies, but there's a question mark as to why. There is also some research demonstrating that GABA can cross the blood-brain barrier at points where there is damage to the barrier itself.
But, there is a form of GABA that is synthesized with a carrier agent that does allow it to pass into the brain. It is called picamilon. It works using niacin (which is vitamin B3). You can read a good piece on GABA here:
wikipedia entry for GABA I'm not enough of a chemist to be able to tell, but I don't think it will work just by taking GABA and B3, you need to take it in the synthesized form available in
Picamilion
The thing I'd add to your regimen is breathing. Set your phone or a timer to go off once every hour (if you can) and for five minutes, just sit somewhere comfortable and count breaths in and out, going for long deep slow belly breaths and controlled exhales. If just doing that on your own sounds like hell to you, there's a great app called Breathe 2 Relax, and if you have a device that can run apps I recommend it. It's a nice visual you can focus on while breathing.