I'm 26 and have been excessively tired since I was 14. I have had a very stressful life and recently got diagnosed with Candida, Adrenal Fatigue and Hypothyrodism. These conditons all cause extreme fatigue if untreated. Look into them, when I found out these problems caused my fatigue I was so relieved as they are all treatable (candida can be cured).
I had been going to the GP for the last 12 years and had tests for anemia, diabetes eveything, the dr's just turned me away as nothing abnormal showed up. My thyroid got in such a bad way that it finally showed up on tests that it is underactive.
Dr's don't know everything, be your own dr and research your symptoms to the max. That's how I found out what is wrong with me, constant research on the internet.
Maybe find an alternative dr, I have a nutrionist that tested me for all my illnesses.