A possibility can be terrifying and still remain extremely improbable.

Fear does not scale with likelihood. A vivid possibility takes up the whole of your imagination whether it has a one percent chance or a fifty percent chance, because the mind pictures the scene rather than the odds. Feeling frightened tells you the image is powerful, not that the event is near.

When a worry grips you, try separating two questions. How bad would it be, and how likely is it. Answer them one at a time. Often the fear stays, but it stops making decisions on your behalf.