This may seem obvious, but just setting a reminder to take a break every 45-90 minutes has helped me step away from the screen, stretch, rest my eyes, breathe, and maybe get some fresh air, all of which seem to help alleviate some stress.
Great closing statement - "Yes, you are a founder, but you are also a partner, son or daughter, friend and student. And you always have the power to decide what your startup’s success or failure will mean in the greater story of your life."
It is important to remember that your startup is a part of your life and not your life. Being grateful and mindful has helped me a lot in my journey with a startup.
There are things beyond your control that you cannot help. Things that were in your control but you were not able to help just leave you with lessons, for your next startup!