Your experience of your state and your feelings about it sound so similar to mine. I think there can be many pieces to the puzzle but hormones and neurotransmitters could most certainly be part of it. Am I not producing enough Dopamine or Adrenalin or whatever substance it is that causes motivation, action? I don't know enough about it.
It is interesting that you have had ME in the past. Do you think that this could have altered your physiology such that your inertia is a permanent hangover from that illness? (Sorry, this is not a helpful or cheery thought, is it?) I know someone who had glandular fever followed by ME in his early twenties. He has never fully recovered and has been on antidepressants ever since (20 years). He was given Seroxat so, if he tries to stop using them, he gets terrible withdrawal symptoms.
I have been prescribed Citalopram but haven't taken it yet. I have had plenty of reason to be sad in the last few years including bereavement but I feel that I have grieved and come to terms with what has happened. I now just feel exhausted, physically and emotionally and not managing very well. The stress has run me down. I am getting pressure from relatives to make more progress and this added stress is making things worse. Are the people around you supportive of you? Or are the antidepressants keeping you calm?
I do feel that I am very much underachieving and this upsets me but the thing I find hardest is the judgement from those close to me who think I will benefit or change from them nagging me all the time. (It has the opposite effect.)
Firsttimemama must be right. Different people have different energy levels - inate and permanent. Since adolescence, I have always been a slow, low energy person. I do what I have to do but slowly and methodically. I do have perfectionist tendencies and anxieties which delay me. I also waste time drifting inbetween tasks - just not very efficient or productive.
You mention that there are days when you are much more productive. I have good and bad says according to my cycle and I have tried tracking this but is more about mood than energy. On a different tack, I have found something that helps: I bought some thermogenic slimming tablets with things like Damiana, Guarana, Ginseng, etc.. in them. You are supposed to take 2 to 4 a day but I haven't taken them regularly (as a slimming aid) because they make me a bit hyper and anxious. I have found that they have an interesting effect though - if I need a 'power' day, I will take one in the morning as they seem to have the effect of making me tick over continuously doing tasks. I find I cannot sit down and do something sedentary like watch tv as I feel frustrated if I am not doing something. To my mind, the fact that these tablets can reliably make me more productive shows that I am not just lazy. But, in themselves, the tablets are not a solution.
I must sound like a right nutter but your OP just rang a few bells with me.