In order to eliminate negative thoughts, it is essential to understand where the thoughts are actually coming from.
Let’s say you are in a room with a rotten fish. It smells bad. You suffer from the smell. However, you know the bad smell is from the rotten fish. You will simply clear the fish and you won’t suffer from the smell any longer.
In the same manner, when you understand how the mind works, and then know the method to clear that mind, you can be free from the negative thoughts and become truly happy.