Not working in a series is like sentencing yourself to an eternity of first days.
First days are hard. You remember them.... first day at school. First day of learning to drive. First day at a new job. Everything is unfamiliar. Awkward. A tinge of excitement for sure, but a bit uncomfortable. If you don't make art in a series then it is like giving yourself an endless stream of first days. You miss out on the chance of benefiting from the experience you gained in the last painting.
I resisted the idea of working in a series for a while. It felt restrictive, limiting opportunities to explore and discover different things. The problem is that there are so very many different things to discover. If you don't stick with something, at least for a little while, you actually make it very hard to learn from what you are doing. And extra hard to avoid feeling discouraged.
Its all because of the learning curve, I think. There is a theory that when you learn something new, you do the most learning at the beginning. That is the hard part. You have so much to learn that if you were to graph your learning over time it would begin with an enormously steep curve and then flatten out with experience. That steep curve is the initial up hill struggle when you are grappling with something different. When the learning curve starts to flatten out it is because you have tucked some of the basics under your belt. You have built up the necessary fundamental knowledge and skill. It was the absence of this when you started that made it feel so hard.
So be kind to yourself. Make art in a series!