Lack of confidence in yourself, your product, and your startup is a surefire recipe for disaster. At the other extreme, too much confidence or arrogance can kill you just as fast. It's always painful when a startup fails, but as a mentor to founders I would hope that you can learn from these failings and not stumble on the same issues:
You probably think these are so obvious that they are clichés. I wish that were true, but I still see them happening every day. The most successful startup founders are never too busy to listen to the market, listen to their advisers, stifle their ego, and enjoy the ride. It's a lot more fun than the alternative.