One thing I have learned through the years reading Seneca, Marcus Aurelius, and other Stoics is that you have to let go of things you cannot control.

An athlete could pour millions of dollars to his training and preparation, but he cannot control the outcome of a game. Nor can he control the possibility of injury.

In business, you might have a comprehensive business plan and detailed execution strategy, but you cannot know for sure if the business will work. Or if the business is existing, you cannot know for sure if the business will still be profitable. New technology might make your business obsolete. Or a new player might take your market share.

Whatever it is you are trying to do, if you let go of things you cannot control, you are bound to be contented with the result. It is out of your control, so why stress about it?