Stress to distress
You have to know the difference between stress and distress, and be able to tell when you move from one to the other - it is incredibly important. As a business owner (or future business owner) you are going to have to handle a ton of stress on multiple levels. It just comes with the territory.
Ordinary stress is perfectly fine and is often an asset to a business owner. Aside from making your hair fall out, stress can make you focus, work hard, and deliver. It cannot be avoided no matter who you are or how 'chilled' you are, so you need to learn how to focus it and make it help you.
The biggest danger with stress is it's ability to escalate into distress. The difference is simple: stress is something that has a solution; distress is something that doesn't. Having a deadline is stress, but if you focus and work you can meet the deadline and that stress goes away. Distress is losing your cool in a traffic jam. There isn't anything you can do but sit and wait for it to clear, but often you will work yourself up into a frenzy and blame everyone from the stupid drivers to the traffic department. You will make a pact to never pay tax again because the traffic department are idiots. Your brain will blow something quite simple completely out of proportion and this is incredibly dangerous. A stressful situation can quickly cause distress if you don't keep a check on it.
There are many things a business owner faces every day that can escalate into distress, and when they do you will become completely useless and defeatest. Don't let it happen! Keep things in the stress zone, identify their end points and work towards those calmy. If you don't you are likely to become a detriment to your company and yourself.