In Control?

I have recently re-read some of the works of Richard Bach and, as far as I can understand it, I, on some level unconscious or not, decide on my level of happiness, mental state, what I do, say, am etc.. i.e.

• If I am miserable it is because I choose to be miserable


• If I am happy it is because I choose to be happy


• If I am depressed it is because I choose to be depressed

So: What mental process or power must I employ to move from one state (miserable, say) to another (happy for example). Is there some sort of "mental switch" that I can flick?