I was in your exact situation. My day job at the time was very fulfilling. I only took the moonlighting gig for money. I hated it - it was a web design job and I'm a research/algorithms/systems person. I ended up spending way more than my billable hours procrastinating and stressing about the gig. My effective wage was shit. I didn't have free time to do stuff I enjoy. I was young. I bailed hard and burned bridges. I…
Were you ever worried about your reputation?
E.g., I got in way, way over my head on a project for a startup once a few years ago. They didn't know what they needed, and I had no true idea how much work was involved. When I couldn't deliver I returned the money (which hurt, but I could do it). The client later came back to me (!) with a much smaller, better-defined project which I executed on well. They became a regular client and have been worth six figures to me over the years. (It did help that they tried another developer in the meantime who also didn't deliver but kept the money and stopped returning calls.)