I struggled with this for a very long time. It's a form of control. We believe that if we can't fully control the future, we are bound to fail. So, naturally, we try to come up with ways to alter the outcome.
Reality is, we can't foresee the future. And that is very scary. The thing is, your thoughts have absolutely no control over good or bad things happening. In fact, if you can learn to accept thoughts as passing images and view your brain as a complex organ, you begin to feel less freaked out. Your brain isn't magic. It can't conquer up some evil or disaster in your life.
It may take awhile to overcome these mental rituals. My suggestion is to slowly begin to allow thoughts to be there and not give them any power. Don't view them as jinxes. Even if ever one of your fears came to life, it would not be because you thought positive or negative about it.
Hope that helps. X