My friends with 3 jobs don't get burnout. They work the hours they're scheduled (or get paid overtime). You perform tasks for a few hours (sometimes very long hours), but then you're done for the day, and the next day you start anew. Some days are worse than others, but in general, work doesn't pile up; you just do what you can each day. The work isn't "carried over" into subsequent days or weeks.

Office work feels more like pushing a boulder uphill, and if you don't make enough progress, the hill's incline seems to get steeper.