A fine cautionary tale on this page. I have the kind of job where there is often just a little more work you could do, a few more appointments you could make.
It was mostly me realising that I had very little time left to spend with my girlfriend that caused me to change how many assignments I would take on at a time.
Now I have learned to insist on not taking on any more work after a certain time (and it really takes insistence to do sometimes).