When you're angry or in a bad mood or upset, it doesn't work very well to try to be positive. Your whole mind-set is against it.
But you can get out of it with a special kind of negative thinking. It is the key technique of Cognitive Therapy (the most effective therapy available, according to over 600 studies).
Here it is: Criticize your own thinking. Be negative about your own mistaken, exaggerated, distorted thoughts. The fact is, when you're upset, your thinking becomes irrational, and your negative (and false) conclusions keep you upset.
Read more: How to Genuinely Feel Less Angry