You probably have chronic fatigue. The way I got over this was an 8-year process. This can be a very debillatating condition. First, determine if you have any food allergies. 2nd, I am still often fighting anemia - which may be caused by bad digestion and not being able to process your food properly. See if you are gluten intolerant or celiac. (I am). When I cut dairy and bread out of my diet, things started turning around.
The best thing to do is work with a naturopath. They are what helped me know what to do when to get over it. Dr. Cheryl Wood in Lynnwood, Dr. Jodi Berg in Everett, and Dr. Bejing Men in Northgate were instrumental in getting me over the chronic fatigue. Believe it or not, acupuncture REALLY helps.
Another thing that helped a LOT was Essiac Tea. You can find out more about this wonderful tea at blessedherbs.com. This is still my huge help when chronic fatigue rears its ugly head again.
The best VITAMIN - for iron and B-12 (you probably also have a B12 deficienty) is Floradix which you can get at Super Supplements. It is a liquid iron that you take, and the best on the market, in my opinion. It doesn't constipate.
The other thing I have to do for my condition is avoid sugar (it REALLY brings me down), avoid white flour. I cannot eat a standard breakfast of waffles or pancakes. If I do, I will have to go right back to bed. Even in the afternoons, if I eat too much sugar, suddenly I have to go back to bed. It really exhausts me. Caffeine will exhaust me as well. If your immune system is run down, caffeine can do a real number. It can make you want to sleep, believe it or not.
I suffered from this condition for 12 years under an MD before I discovered diet and Naturopaths were the only road out. I had it so bad I wanted to die. This is an autoimmune disorder. Cleansings help. Diet helps.
God bless you on your road to recovery. Prayer and meditation helps immensely. Being in the monastery for a few days, focusing on prayer only gave me some immediate healing. Being stressed can also cause an immense amount of tiredness. When I'm stressed, I sleep as well.