Stress is sometimes what happens to us. But too frequently, it’s about what our minds think is happening, and that’s a problem. The problem is that too frequently you are far more hurt by your thoughts of what happened, and not by what actually happened. For example, most of us too frequently overreact when someone points out a flaw in something we did. The criticism may contain one useful fact, but our minds can turn it into a judgment about our entire worth. We replay it, imagine worse outcomes, and focus on everything that might go wrong. You cannot always choose the first thought that enters your mind, but most of the time, you can choose which thought gets your attention next. That is the useful idea behind this quote.
William James wrote extensively about attention. In The Principles of Psychology in 1890, he described attention as selecting one thing from several. In other words, your mind cannot focus equally on everything at once. Think about how craze it would be if every sound all day long got equal attention. That would drive you crazy! The same goes for your internal thoughts. You get to choose.
Stress, angst. That anxiety is a cue to you that you need to slow down and look more carefully at what you are doing. Not every anxiety is an ethical problem, but when the tension comes from a difficult choice or uncertainty, pause. Listen to the guidance around you. Clarify what is actually your responsibility. Think through your intent, likely results, and who may be affected. Anxiety is your mind telling you that the easy answer is not enough.
You may not control every thought that appears, but you can get better at deciding which ones deserve your time.