Loss of mental energy

I used to be such an avid reader in the past. There were many days when I would study for as many as 12 hours a day. I felt that I had unlimited amount of energy. Since five years ago, my mental energy has been going down. Although I do not feel depressed, I have lost my mental energy as if my brain has permanently run out of energy. I take vitamin pills and do aromotherapy but they do not make a big difference. I am now 35. where has my magnificent memory and mental energy gone? I had an MRI done but there was nothing. How can I refuel my brain and go back to studying for 12 hours a day again? Please advise as to my condition.

Thank you
Reza

As you get older, you are able to compensate for loss of the working capacity of younger people, through larger experience and knowledge.

To study 12 hours a day sounds very much to me, few people can do that even when they are young.

If you have experienced a sudden loss of energy and capacity, this might however be a symptom of depression.