What is one way for a startup founder to stay happy no matter what?
As a start-up you want to have a mission you are working towards and trying to accomplish, but you should also remember that achieving the goal is not 100% within your control. You are just one factor, one member of the team, one part of the strategy. You're only a contributor to achieving that goal. And even when you achieve the goal, it is not going to make you happy.
You need to find a way to work without actually wrapping up your entire ego and identity in accomplishing this one goal.