One of my great ‘life lessons’ came about from reading a quotation which I think is attributed to the Dalai Lama.
“If you are in control of a situation, there is no need to worry. If you are not in control, there is no point worrying. So, why worry?”
Similarly in business, there are things you can control (or manage) and things outside of your control (external factors). You need to be aware of the external factors (how the environment is changing, what your competitors are doing, and so on) but you should concentrate your activity on those things you can manage.