> I’m not sure on the root cause of my burnout as I work a fairly regular 40 hour work week
The forty-hour work week measures how long people spend at work, not the time they spend working.
I bill the latter. The clock stops if I take a break, however brief. That means eight hours in a day don’t contain eight hours of billable work, but it does give me flexibility over my time: I work when it’s productive—which can vary with the day—not when the clock dictates. You may be doing the same.
Even if you charge differently, the typical forty hours (which companies and countries are experimenting with shortening[1]) don’t take you into consideration. It may be that you need fewer working hours to function, and that’s OK. The people who brag about sleeping less or working more than average are not worthy of admiration.