You replaced your boss with a customer. No real change there; You're still at the mercy of someone. I'd say you're a dumbass for not realizing this, but I'll decline to as we're supposed to keep a higher standard here on HN.
If you are the owner, it doesn't matter if one or two customers are hard to deal with. All you do is stop selling to them, and you will most likely still be in business.
1. Already sold your company.
2. Didn't really contribute to its founding.
This is like an employee accepting a $10/hour contract, and then complaining later that it was too low.