"Simply because my clients were confused. They would let the product stay alive for a couple of weeks, without marketing it, expect it to convert well and become rich overnight. And when that didn't happen, they would kill it."

This is a lesson for countless others who start with building products for clients on their own (thanks for articulating the situation). Milestones are the key, as well as sticking to the stringent "No further development without milestone payment" policy.