The most critical part of launching a successful start-up can also be the hardest — learning how to navigate calmly forward in a fast-moving, ever-changing environment.

“I have learned, as an entrepreneur, the most important skill you have to have is you have to be an optimist and pessimist at the same time,” said Stephanie Tilenius, former Google VP and founder of Vida, an app that pairs you with 24/7 access to a health coach.

“The hardest thing is the psychology of it,” she said.

Tilenius said she has to be both relentlessly optimistic about her long-term goals for the company and brutally honest about what’s not working in the day-to-day operations.