The problem is that the inner consciousness, the part of ourselves with which metaphysicians believe we create our reality, thinks in concrete terms, and negation is an abstract concept. Thoughts are represented deep in our minds as sights, sounds, feelings, tastes, smells, movements and many other subtle senses. There is no way to picture a "not" something. If I ask you to not think of a purple rhinoceros, you must think of a purple rhinoceros at least for a moment, just in order to understand what I said.
So, when I am thinking over and over again "Please don't let this happen to me!" I am actually focusing my energy and attention on an image of the very thing I don't want to happen. Focusing our energy and attention on something is an effective way to draw it into our lives. Much better to focus your energy and attention on what you do want. This is called an affirmation.
Positive commands, or affirmations, to ourselves (as well as to suggestible others) do work, but a word of caution - you may not fully comprehend what you are asking for. I have many times had the experience of writing an affirmation or meditating powerfully on what I want, and then seeing chaos break out in my life. My affirmation has begun a process of bringing me to what I want. That process may consist of confronting everything within me and around me that stands in my way! Or I may be confronted with the true meaning to me of what it is I think I want. Stick with affirmations if this happens to you, letting them evolve as necessary. Once you've begun a process like this, the only way out is through.
If you navigate away from what you don't want, whether it's something in you or some outside force, "what you don't want" has become your navigational aid, the thing you are steering by. Anything that you hold in your mind will draw you to it, even if you think you are steering away from it. Make a practice of thinking about negatives only long enough to know that's not what you want. Hold the positive in your mind for guidance. Remember that "don't think about that" is still a negative command to yourself. Just gently guide yourself back to "think about this". And the next time you see a child headed for a puddle, try saying "be sure to step around that puddle!'