Have you read Bhante's booklet? Perhaps you are trying to practice a Samatha meditation. Try Satipattahana meditation for a change. The section dealing with the thoughts is called Cittanupassana. In Cittanupassana, you are supposed to be mindful of the thoughts coming up instead of trying to stop them. They can be thoughts about the past/future or hateful thoughts, greedy thoughts, sad thoughts etc. You just need to keep reminding yourself of what's happening instead of trying to stop them.