Different drugs act differently with different people. Hence it is advisible to take guidance from the treating doctor. Male deer exercise and kegel exercises are good for sexual vigour. Google these phrases.
Zoloft (or any SSRI for that matter) is notorious for making it difficult to ejaculate. As far as the erection is concerned, I suspect this is not related to the bupropion. Have you tried one of the PDE5 inhibitors (i.e., Viagra, Levitra, Cialis)?
After taking 400mgs of Bupropion. I'm down to 200mgs!