I agree with Elizabeth that we all get foreboding thoughts on occasion. How we handle them though is what's important. Take for instance if someone is late coming home. Your mind will explore all the possibilities and then sort out which ones are realistic and probable. The "What can happen?" experience allows us to do some preventive planning, and in that sense it is useful.
If you listen to your your worst thought scenarios and take then to heart, then do challenge them. Want to tell us what kinds of thoughts you mean, and how you handle them at the present? We may be able to offer some suggestions.