Dear alison,
Good question.
You may replace the thoughts with positive thoughts HOWEVER, you cant stop your thoughts.
The more you try to control yr mind, the more rebellious it would be.
If you listen your mind well enough, you will find that it's full of positive and also negative thoughts, they never get out of our minds.
But lucky is, we can calm our mind down by meditation. The more calm we are, the better we can understand our mind. From that, we realise that letting go of thoughts is the best way instead of replace them with positive thoughts.
"Mind is very bad master, but it is excellent slave"
Got it?