Challenge
One of the biggest challenges has always been to keep believing even when bad news has just been received. I read and listen to a lot of positive stuff on perseverance and persistence by the likes of Tony Robbins, Brian Tracy and the like. I need my daily dose of positivism from reading to keep me strong as running your own business can be very testing.
They do it for me, and I genuinely believe that adversity is a test to see what you do next. The majority of people often back off but success is often just beyond adversity I have always found.