So tired all the time!....?

I am a healthy 28 year old girl, but I feel tired and lifeless all the time! I have decent sleep and I go to the gym quite regularly. Its been happening for ages and I've been the doctor twice but nothing seems wrong. Its affecting my work tons because I don't have the energy to do anything and sometimes just stare at my screen and just want to put my head on the desk! Is there anything that can give me a boost or something?? In an ideal world, there would be a pill (legal pill that is!!!) you could take that would instantly wake you up and make you feel euphoric!! The closest I've felt to that is after taking nurofen plus, but, hey, can't really take those all the time!! Please, can anybody help?