Thinking that one can stop thinking is a common belief that many people hold when they come to a mindfulness meditation class. The short answer is that it is in the mind’s nature to think and, for this reason, the thoughts can’t be stopped. What can be done is that one can learn to work with the thoughts and change how we relate to them.
Here are some ideas for the “how to”.
When a thought arises and you realize you are paying attention to it rather than the breath, the first strategy would be to try to let go of the thought and gently and kindly return your attention back to the breath. You can expect that you may have to do this over and over and over. While this can be discouraging, see it as an opportunity to practice patience and determination. In time, as you learn to pay attention to other aspects of your experience, the dominance of the thinking mind will reduce.
In the event that the thoughts continue to capture your attention and it continues to be difficult to have the majority of attention focused on the breath, try this. Expand the field of awareness of the breath to include your thoughts. Consider taking the stance of an observer and entertain the idea that your thoughts could be represented by a river in the mind. Depending upon the characteristics of the thoughts, it may seem like you may have anything from a raging rapid to an almost dry stream running through your mind. In the image, place yourself on the bank of the river and imagine you are observing your thoughts from this place. Make sure you place yourself high enough on the bank in order to contact a place of calmness and safety, well out of the way of the currents of the mind.