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