One of Stoicism's basic premises is the practice of focusing our energy only on things over which we have control. For most of us, this will be much easier said than done, but consider this:
If there is nothing you can do to influence a situation, why waste your limited mental energy worrying about it?
Said another way, if you knew that the outcome would be the same no matter how much you worried about something, would you bother to worry about it?
Think about the weather, something over which none of us has any control. It's going to rain, or it's not.
Worrying about it won't change anything.
But, if you can train your focus and energy only on things you can influence, you'll inevitably see results.
The image below contains two nested circles. The larger one represents EVERYTHING in your life. Within it, a smaller circle represents only the things that you can influence. Everything else is outside your control.
Anytime a challenge presents itself, think about which circle it falls into, and determine how much control you have over the outcome. Then act accordingly.
One final thought: Even when we have no control over a situation, we can always control our preparation and response to the problem. The presence of rain, for example, is outside of my control, but the decision to bring a raincoat is not. And when I have a raincoat, I have no reason to worry about the rain.