"How do you know if you're in over your head in a healthy way as compared to an unhealthy one?  One entrepreneur gave me a good rule of thumb for this just yesterday.  He suggested that entrepreneurs follow an 80/20 rule - they should always feel in command of 80% of the business, but feel way over their head 20% of the time.  It's that 20% stretch that makes it fun and challenging. "

--Boston VC Jeff Bussgang, writing at Seeing Both Sides, about the stretch running a company can create--he's got some good coping suggestions as well.