When you are starting your own business, an important thing to learn is to manage your emotions and keep calm in every situation - good or bad.
There will be days when something great will happen and you'll find yourself in the sky. The same day, in the evening, the sky will break and fall on your head. Understand that this is quite normal when you are starting your own business. Learn to be neutral in both situations.
How to keep calm in extremely negative situations?
1.) Think about the larger picture. When you are starting your business, you're doing it for the next 10 or 20 or more years. Ask yourself - "will this matter after 10 years?" If the answer is no, then no point in worrying about it.
2.) Distract yourself from the situation. Listen to a podcast. Read a book. Talk to your parents/friends. Go for a trip. Play a sport. Just avoid thinking about the situation for a day and you'd be surprised how much better you'd feel the next day.
You should not let the negative situation get into your head and cause you to be stressed out. At the end of the day, there is nothing more expensive than your mental health. Having said that, make sure to learn from the situation so that you don't end up in the same situation again.
Summary: deal with the situation with a calm mind and learn how to not repeat it.