I worked in the ad industry for years and made good money doing it. I now work, unpaid, on a FOSS passion project.
What I would advise is treating each job as a college course which you also happen to be paid for. For me, this meant not allowing myself to become emotionally invested in the work, taking good notes about what I learned each day, and looking for a new job (typically with a raise) once those notes were sparse for too long.
