Christians make a very serious mistake in their thinking about "thinking". We want to believe that we cannot help what we think. The truth, however, is not only that we can help what we think but also that we must take full responsibility for our thoughts.
Jesus said, "Don't let your heart be troubled". Wouldn't it have been helpful if He had said, "I won't let your heart be troubled"? Psalm 37 tells us, "Fret not, it only tends to evil". Can we stop worrying and fretting? Of course! Can we stop doubting? Can we stop daydreaming? Can we stop plotting evil? Absolutely!
Now don't get me wrong. I am not saying that it will be easy. Thinking habits are as difficult to break as any other habit. But it can be done. And the first step is to acknowledge that it is possible.