Shun idleness. It is a rust that attaches itself to the most brilliant metals.
—Voltaire (1724, by Nicolas de Largillière; Public Domain)
It took me a number of obstinate decades to figure this out: When our minds get caught up in dwelling about what might have been, or angst over what might be, nothing will shake us out of this state like action. Forget what has happened in the past; focus on the desired destination, and immediately begin some task that will bring you closer to that point. It is amazing how effective this is at changing our state of mind.