In my experience, when added to vitamin D and magnesium, the result of collagen peptides was remarkable. After an initial several days of fatigue, my circulation and level of energy improved. Areas of skin on my face turned red and then cleared to new skin. Friends and family described me as looking much healthier, which was how I felt.
Another line of inquiry would be to check for possible GI issues like celiac disease or food intolerances. They can lead to vitamin and mineral deficiencies that have a wide range of effects.
You might want to try an NADH supplement and alpha lipoic acid. They can be helpful with fatigue type disorders.
I did take Alpha lipoic acid- (nerve damage in feet and hand due to severed nerve)- I didn’t see much difference- i have not tried the collagen liquid though- I will check that out- I agree it might be Chronic Fatigue- due to symptoms- Especially the PEM, which is a huge issue- days of recovery after doing hardly anything- and it hits 2 days after, not right away- which is classic PEM I guess - but could be a number of other things too-
Haven’t tried NADH either- got it in my amazon list though-
I have been tested for celiac- I have GI disease- so they keep track of that pretty well-