I can imagine how tired you must be if your iron level is only at 6 right now. You say the doctors cannot tell you why your body does not absorb iron.
If you care to share more, please let us know what types of diagnostic tests you have had. Has it been just blood work or has an upper endoscopy been done?
While I am not a medical professional, I would like to share the experience of a friend of mine who had the same symptoms of low iron level and terrible fatigue. They found that she had celiac disease. She now follows a gluten-free diet and is doing quite well.