Fatigue and loss of energy

hi, i had mono about 7 years ago, but i still experience fatigue and loss of energy. i do remember that i was so energetic before, but now i fall into depression so easily.. i was told by the doctors to keep myself fit by following a healthy lifestyle so i've been trying to follow their instructions, which really has helped a lot. however, i still have those weird feelings.. what could help me, is there any drug which could deal with it, i've heard something about Ritalin...