Odds of Hanging On
Let's say the stress doesn't get to you. Let's say you can play nice with everyone and keep everyone happy. Let's say you can even come in on time and under budget.
You still have absolutely no control over whether you can keep working.
Who does? The movie-going public. It is up to millions of random people that you've never met to decide to drop the 12 dollars and come out to a loud, smelly theater with a sticky floor and screaming kids and people who answer their cell phone to see your movie. Getting really, really good can help stack the odds in your favor. But the best advice you can get as a director is "invest your money well."