I too suffer from long term fatigue - I have cerebral palsy - so I can empathise
Sometimes it takes a lot of work to find, and then treat, the problem - it's been over 10 years for me and noone still can give me a definitive answer.
but things have improved markedly since I stopped drinking and smoking...though it took the best part of a year tho for me to do so.
Keep looking for the answer
D