This is a common misconception. The intent of a mindfulness meditation is not to stop your thoughts, but rather, to be aware of thoughts when they arise. Thoughts are not bad and if you resist them, you make them stronger. So don't. Keep returning to your breathing.  Over time, your mind will become calmer all by itself. With practice, you can experience times when you have no thoughts whatsoever. In those moments, you are very close to something special. That's another class.