It dawns on you: if we get any more than a few customers we might not survive!
You are already working crazy hours and don't have the staff (nor the ability to quickly pull in new staff) to handle an influx of customers; to install, support, and maintain. I'm talking about a larger niche product here, not punching out widgets from a machine where scalability was designed in from the start. Maybe you didn't design the product properly and it's a little too flakey for your liking.
What do you do? How do you stop your mind racing through all the different scenarios, success and failure? Meditate? Convince yourself to worry about that problem if and when it appears? How?
Seriously! There is a type of personality out there, me included, who think way too much and have trouble loosening up and enjoying the ride.