The easiest way to change your mood or your state of mind is to change your attitude about it. Close your eyes and remember how you feel when you are doing something you love. Did your posture change? What about your breathing? Does your body seem lighter?
When we are engaged in something that makes us happy, we become more like children, we get filled with the joy and the wonder of that moment. And then, the moment changes, because it always does. As a result, the beautiful and the light qualities of joy go away immediately. To retain those positive qualities for as long as you wish, you have to learn to recreate how they feel inside your body.
How does joy feel like? Close your eyes and imagine. Deepen your breathing, drop your shoulders down, lift your chest up, raise your head up high, allow your walk to be lighter and watch your attitude change right then and there. Watch your mental state becoming lighter,more expansive, happier, more luminous.
This exercise can be applied to any feelings. Next time you catch yourself experiencing an emotion, note where in your body are you feeling it. Note all sensations and physical manifestations of it. Note your breath, your body. Then, whenever you'd like to recreate that emotion or attitude, close your eyes and actively and purposely bring on its physical characteristics. You can experiment with feelings, like joy or freedom, or mental states, like creativity.
You can bring this "attitude play" with you to the yoga mat, invoking different states in different yoga poses and noting where in your physical body are you feeling them the most. Where in your body do you feel pain? Do you typically feel pain in the same areas? It usually indicates "stuck" energy patterns. By releasing the attitude, which causes those areas of your body to tense up, you can release your aches and pains, both physically and emotionally. Most importantly, don't forget to play: this exercise is fun!