I've been fatigued since I was a young teenager and this is one of the many reasons I want to ctb. I really feel for those in the same situation, especially if you've been failed by current medical science and are stuck in the trap of chronic illness with no end in sight.
If you haven't already gotten B vitamins, vitamin D, iron, thyroid panel, A1C, inflammation markers, celiac disease antibodies, autoimmunity antibodies (for things like Lupus, Hashimotos, Sjorgens etc) tested, it's a good place to start.
However, once you've had all of those tests done repeatedly, including an expensive brain MRI too, and you get told there's nothing you can do except try the same antidepressants you've taken a billion times already, the pursuit for relief feels fruitless. To be told at 21 years old that there's nothing more that can be done for you, is life ruining. No one understands either. You will most likely be gaslighted and told you're secretly depressed and in denial of it, even if the fatigue is persistent and happens regardless of mood.
There are many causes of fatigue and not all of them can be detected with the current biomarkers and tests we have at our displsal, so our current medical industry is completely rubbish at treating such a nebulous and unspecific symptom. I was always a pretty palor and sickly kid, and went through a great deal of abuse and trauma which I think permanently damaged my immune system and my body as a whole.
I think it is incredibly common for abused children (and adults) to develop ideopathic chronic fatigue. Even moreso if you have other issues, like IBS, pain, autism, pre-existing mental health issues, and so on. When your body is put through severe stress like that for ages, eventually some bits and pieces are going to wear down or try to compensate for the heavy energy burden of managing stress.
No one really understands how systemic fatigue truly is, and how deeply it effects one's overall health and wellbeing. Energy is the currency of life. If you have no energy to spend, you cannot truly live life to the fullest. You can't push through tiredness either. Getting chronic fatigue made me stupid. It ruined my eyesight, making my vision permanently blurred, unfocused, and slurry, and I am gaslighted by doctors and told that my glasses perscription is fine, so there must not be an issue. It made me stupid and gave me attention, concentration, and processing issues that not even 5 different stimulant drugs could fix.
It ruined my already limited ability to socialise or be creative, which was already in the mud due to being autistic. Having a blank mind all the time is horrible. I have to see everyone else my age doing sports, traveling, going on long nights out, and living life, while I often am confined to my room because I know I will end up tired, in pain, and many times have nothing to say. I'm so slow when walking due to my fatigued, heavy limbs that people will push past me. I wake up with my limbs numb and tingling every morning, and everything unrefeshing and blurry as always.
My last resort is taking LDN that I'm compounding myself. It's an experimental treatment that many are using for chronic fatigue. If that doesn't work, which it probably won't, I consider myself shit out of luck.