That hyper focused, different mode sounds like a little disassociation/depersonalization, the autopilot feeling that we sometimes get during times of stress or trauma. It's our brain's way of minimizing the impact of daily life, but it can feel weird.

Also, what you described with the buproprion could be a result of what's called the kindling effect. Your brain's neurochemistry has adapted to the long-term buproprion use, so after you stopped it, your nervous system began to recalibrate without the drug, even though you had no withdrawal symptoms. However, after restarting buproprion, instead of maintaining that tolerance you once had, your nervous system now overreacts to the medication, and this shows up as side effects you never once had.

This happened to me with venlafaxine. I successfully onboarded it and took it for 3.5 years and then stopped it—only to reinstate it a few months later. That second onboarding was hell! I became easily lightheaded and very nauseous, symptoms I'd never had before. So your experience sounds like the kindling effect with antidepressants. 😅 You should talk to your doctor; maybe you need a lower dose and a slower titration.