It is so easy to do that bit about imagining the worst case scenarios. We are almost taught to do that in most western societies and Americans are really good at it. Plus, since those things really are possible, how can you not think of them?
I think the main thing is, don't allow yourself to focus on them, and instead focus on what you can do right this very minute to help yourself. When I start doing worst case scenarios, I go for a walk and look at nature, play with my dogs, read a book or watch a movie, get on this forum and try to be helpful to someone, or in some other healthy way distract myself from those thoughts. When they come back, I do the same thing. It's good also to focus on the best case scenario if you can. So, I do this: Well, it could turn out the very worst way, but hey....it's just as likely to turn out the best way." I just keep thinking that, every time the worst scenario come to my mind.
You can't really just shut it off, or make it not come into your mind, but you can definitely add something onto it to counteract it, and you can distract yourself. I do a combination of those things, and it helps a lot. I also remind myself that worrying does absolutely nothing except hurt me and make the bad thing more likely. That usually helps, too.