In the moment you find yourself stressed, tense, worried, irritable, or angry -- when you find yourself in a negative state, thinking about what to do about your negativity -- all you have to do to break free is realize that you're in the wrong place. Don't think about what to do with where you are. Admit to yourself you're in the wrong place, and then... just don't go there.
You may, at any moment that you wish, become aware of yourself sufficiently enough to know where you are inwardly. When you find yourself living a resentment over again, all you have to do is see that you're surrounded by thoughts and feelings that were produced by your resistance to a remembered event. That's all you have to do. Who made you remember what you now wish hadn't happened to you? Who made you picture the person you resent? Who brought up the failure from your past? Who did any of that in the moment you're sitting by yourself, or driving in your car, or eating your cup of coffee.