Did the doctor also run a CBC (complete blood count) to check you for anemia?
Do you get regular sleep (go to sleep at a reasonable hour, around the same time every night, and get at least 7 hours of sleep?) That is truly the key to getting rid of severe fatigue, IF you do not have a good sleep schedule.
Other things to consider: mild dehydration could cause major fatigue. Drink PLENTY of water all throughout the day. Eating a lot of refined sugar, not enough protein and healthy fats, not getting enough B vitamins... these could all play a role.
Do you think you are depressed?
Stay away from alcohol, which acts as a depressent.
Wellbutrin can cause more fatigue and/or weight gain in some people, but it's good to give it a chance. When you do go off of it someday, be sure to wean yourself off slowly to avoid side effects.
I hope you start to feel better soon.