I've known not to mix business and pleasure with personal projects for ages, but as a result of needing to do commissions for money, it made it harder and harder to enjoy producing art. My reasons for doing art became unfocused and unclear, and it lead to a lot of resentment over time. I've been attempting to remember this rule by asking myself what I want from art before I start making it. If the answer is unclear, I allow myself a break to consider my thoughts.
This has helped clarify an issue I've had with my own art that lead to burnout! I'm glad to see someone else who's thought about this, and put their thoughts out there for others.