I've taken anti-depressants on and off for years. I was on Bupropion for over a year more recently, as well as Lexapro at the same time. I was on one for depression (Lexapro) and the other (Bupropion) to counter-act the side-effects from the first. I recently switched from those as I think they had reached their "shelf life" in my body. And, I just went off my most recent drug, Cymbalta, because I want to try to get pregnant soon. I'm currently going through Cymbalta withdrawals. Ugh.
First off, you may need to give the drug time to take effect. This can take a couple weeks - possibly even a month. Give it a little time, and if you continue to feel tired, it could be that Bupropion may not be a good fit for your body. Anti-depressants work well in general, but every person's body is different, and some people have more side effects than benefits. Talk to your doctor about your symptoms and see if there is something else you can try - there are lots out there now - Lexapro, Paxil, Prozac, Cymbalta, etc. Above all else, keep your doctor informed of how you're feeling. It make take a switch or two to new meds, but they should be able to find something that will click with your chemistry. Good luck!