Cloudscape

If only one understands one's own feelings, one has full control over them. Actually, this is so for anything at all in the universe, but the miracle is that to control one's mind, one doesn't need to do anything in the physical world. One can tweak one's neurochemistry by using antidepressants, but it can just as well be done through sheer will.
The thing is, most people can't control their feelings because they don't understand them -- often they don't even know how they feel. The more one is aware of one's feelings, the easier it will be to harmonize them.
Often people will try to be happy and find that they fail. This is because happiness, like all higher feelings, is very complex. An emotional state isn't just one color -- it's a painting of a thousand hues; to achieve an emotional state, then, is like making a painting. Usually, one can't just be happy like that - one is happy about something. Even when it has a physical cause such as drugs, emotion always has a reason: an object. Do not try to be happy: try to enjoy.
One cannot fill one's mind with happiness without filling each sensation and each thought with happiness - for they are what compose your mind. Visualize that you were filling everything around you with energy, every sight and sound and smell and touch, and it will in turn fill you with energy -- for if all these things are encompassed in one's brain, they are part of your being.