I've been reading a lot about mastering thought recently and it is true that we create our own heaven and hell within our own minds. From time to time, we all catch ourselves obsessing over something that has happened or someone who has upset us in some way. But the moment has passed. The only way it can still hurt us is within our own minds, by continuing to attach negative emotion to it.
Try this next time you have a recurring negative thought - ask yourself these four questions:
1. Is it true? (Yes or no. If no, move to 3)
2. Can you absolutely know that it's true? (Yes or no)
3. How do you react, what happens, when you believe that thought?
4. Who would you be without the thought.
This short exercise comes from "The Work" by Byron Katie. It helps us to become willing to let go of negative beliefs. It's worked for me. Give it a try sometime and see what happens.