Started as a solo developer, spent 10+ years to grow it into a small business with 40 employees.
The hardest lesson for me was that at the crux of it, business is all about scarcity. There is no other magic out there. A great question to ask yourself is: How many people want what I sell and how many people are already serving it.
Business is about being in the right market with the right product, and then differentiating against all other players in the market. Once you figure this out, you will solve most of the operational problems.