Mediocre mood is an energy killer. You can change it with a simple imagination: imagine yourself taking a little break from your current situation, as if you were going to the movies and totally forget about yourself. Give yourself 2 minutes to visualize the theatres, the movie and the easiness you feel when you are in the movies. Fast forward your imagination to the end of the movie, see yourself leaving the cinema peacefully and totally relaxed. Take the easiness feeling along into your here and now. Whenever your thoughts remind you that you were in a bad mood originally, just smile and switch to the movie-experience.
Games like that can be helpful, lift your overall wellbeing. You learn to overcome wasting thinking habits that pull you down and replace them with new thoughts. However, if you suffer from serious low feelings over a longer period of time, you must realize that something inside of you might want to be admitted. Check this article https://walkingguru.wordpress.com/depression/ if you suffer from a depression, you need to get help to solve the issue rather fake a good mood! If depression becomes chronic it is very hard to get rid of it.