Here is a way to stop negative thoughts:
Focus your mind on your heart.
Breathe in deeply, keeping your focus on your heart. Really concentrate on feeling love in your heart. Breathe out holding your focus on your heart and feeling the love in your heart. Repeat this a total of seven times.

When you do this correctly, you will feel a very big difference in your mind and your whole body.
You will feel more peaceful, you will feel lighter, and you will have stopped those negative thoughts.