When you go to a movie and watch the actor in the film get an incurable disease, it doesn't bother you. If he loses his job and goes bankrupt, it's no big deal to you. You're too busy eating popcorn and slurping on a Coke to feel his pain! But when you watch the imaginary "movies in your mind" you are the one in the movie. You watch yourself get the incurable disease. You see yourself losing your job and going bankrupt. Have you ever noticed how the "movies in your mind" never have a happy ending?
God has told us in 2 Cor. 10:5 how to stop the movies from playing in our minds. "Casting down imaginations and every high thing that exalts itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ." We REMOVE the fearful thoughts of the future by casting them down in Jesus' name, and then we REPLACE them with encouraging thoughts that God wants us to dwell upon.
It's a simple two-step process, but you must carry out both actions--remove and replace. And if you'll do that, you'll start seeing happy endings when you think of the future.