been sitting with this question for six months and i think i finally have an answer, or at least my answer.
bad job: you dread mondays but feel okay by wednesday. you complain about your manager but you still care about the output. you can turn off at 6pm. you'd take the same job at a better company.
burnout: it doesn't matter what day it is. you've stopped caring whether the thing you built is good. you have the emotional bandwidth of a depleted phone battery and no charger in sight. you'd turn down the same role at a company you actually respect because you don't trust yourself to perform.
that last one is the tell for me. when i stopped believing i could do my job well, that wasn't about my employer. that was burnout.
i'm an SRE. the pager is relentless. we had 11 months of P0s in a row, which is insane but also just. tech. what crept up on me wasn't the incidents themselves. it was that i stopped doing anything after an incident. no blameless retro, no fixing the alerting, just. closed the ticket and waited for the next one. that detachment is what i should have caught.
things that helped me distinguish them: take a week off and notice if you feel relief or nothing. relief = bad job. nothing = burnout. ask if you'd feel differently in a different role at a different company. if the answer is genuinely no, that's burnout. track how many times in a week you feel anything about your work. pride, frustration, investment. if it's zero, something is wrong.
the tricky middle ground is when a bad job CAUSES burnout. that's real. but the fix is different. quitting doesn't restore capacity if the capacity itself is gone.
i stayed too long trying to fix the job when i should have been restoring myself. six months of therapy, sleep hygiene, and genuinely boring weekends later: i can care about infrastructure again. didn't expect to miss it.