I don't have any advice for you but wanted to respond and say I have the same mental fatigue symptoms and it is truly horrible. My physical fatigue is there but not real bad. Ican and do walk lots.
I am female so don't have the testosterone issues you do but find that interesting.
You also mentioned severe GI issues. I do as well - in my case burping and bloating. I do feel they are the cause of my symptoms as a low carb diet years ago resolved all symptoms (it doens't now).
I do know of a woman whowas spacey all the time and a seriees of fasting resolved the issue ... over time.
I'm going to start doing intermittent fasting combind iwth bowel cleaninsg with psyllium, mainly because I have not tried this yet and it has helped others that I know tremendously.