Our thinking comes from beliefs we have invested in. Thoughts are like echoes of old beliefs. Attempting to replace negative thoughts with new ones doesn't work very well because you aren't removing the source of the negative thoughts. To really take out the negative thoughts and end the incessant thinking you have to address the core beliefs behind the thoughts.
For background read the article
The Voice in Your Head If you are serious about this then do the exercises in the Self Mastery course on my site. It will walk you through some practical steps for quieting the mind. It's a much faster path than meditation.