Something in the brain just doesn't quite work right I would suspect and fatigue is a cycle that is hard to break once in it May I? Some ANECDATA for you!
I contracted ME/CFS after a stomach bug. I was 20. 2 years bed/floor bound then I managed a few years of sedentary living, working at a desk. I had a few other issues and the ME/CFS had lessened enough for me to sideline it. During that time I tried to lose weight by taking a variety of exercise classes, I walke and cycled everywhere
At 30 I was made redundant and went to University to do a Sport Science degree and Masters. I supported myself by teaching fitness classes. Lots of them, up to 20 a week. As long as I was slow and careful during the day and do nothing over the weekend I could teach 2 - 3 classes during the late afternoon, into the evening. I'd sleep like a log and drag myself to the library as soon as it opened. If I had a lie in I would sleep the whole day through.
By my mid 40s I had crashed again. I was nigh on narcolpetic. Was then diagnosed with pernicious/megalobalstic anaemia that was masking CFS (or vice versa). My day job at that time was working in a community charity, exercise, food, mental health, a job I loved. I didn't want to stop. But I had to. Could not have continued however hard I tried.
I took a course to start teaching in FE... seemed easier, I could sit down a lot. I went from being active, teaching exericse classes to being a desk bound frump, unable to move at more than a snail's pace. And I know I was lucky to have been able to work at all! All that exercise should have been far more harmful than it was.
But no amount of wishful thinking or brain training will make it possible for me to exercise like that again. Gentle dog walking is knackering - again I am lucky I can do that, and it took 10 years before I felt well enough to get a dog and be able to walk it often enough.
I cannot think myself into being able to exercise. I can only pace myself, do what I can when I can and sit down, do nothing when I need to. I know others have it far worse, others less. I have had better and worse periods of time. NOTHING I could think would have changed those periods of time!
Whatever you might find inetresting about the connection between ME and mental health you have GOT to stop touting mental issues as the cause.
As I sit here, in a good period, if I stop and think about my body, the signals it is sending me, I can feel the glands in my neck swelling.. I heed those, I kow they will put me into bed if I don't. My head is muzzy, my thinkig not quite as clear as it might be. I heed that, I could lose track of what I am doing, what I have planned for the day. I need to re-evaluate... what should I drop from my list? My feet are cold, so I have put on extra socks and turned the heating up, the cold will also put me to bed if I don't counter it.
There is more, and I have contingency plans for all of it. Unless you have ME / CFS or a similarly draining chronic condition yu cannot imagine how patronising and downright fucking antagonising "It's in your head' type comments are. It isn't in my head.... I get mental health swings because of the utter fucking hideousness of being so bloody tired all the time!
Anyway... off to walk the dog! SLOWLY!