Hi prefabsprout,
Perhaps you have a depression right now. It is good that you will follow up with your GP about your fatigue. Fatigue and depression often go together but they are not the same thing. I got a serious illness that was masked under an existing depression, the illness went untreated until I collapsed.
On the other hand, I don't know you but I do not consider you a career failure just because you are at a low paid low range job right now. I am not taking responsibility from your own choices, etc but I invite you to have a wider perspective. Unavailability of better jobs is not all your fault. There are a lot of problems in the job market today. And this is not going to improve unless something is done at higher levels. And to me it will take the commitment of all of us and not only of the ruling class. We need to let them know that this is an unbearable situation. But I am not saying you need to become a revolutionary or an advocate. I am just saying that perhaps you need to put things in context when evaluating yourself. Because of depression I often torture myself by thinking of bad decisions and lost opportunities but I don't have a balanced view as depression wants me to sink in.
Anyway, I wish you the best with all your doctors' appointments and hope you can get the necessary help for you to feel better.