I am biased against Slavoj Žižek because I'm not very fond of him.
I disagree with happiness not being important. It is important, but it is not something that you can have all of the time, because human mood, emotional state and the experience itself is always a wave, going up and down, constant fluctuation. A person needs to learn to identify where in the curve they are and not allow the curve to rule their life. Once your able to be aware of the curve you can start taking steps to decrease the down trend of the curve so that you can spend more time in the up trend.
Humans notice change - diff, if you will. Meaning if you were to be in absolute happiness for extended period of time, your frame of reference would slowly fade and you would no longer know what is happiness for you.
That's why, it is my personal opinion, that you should just sit down, drink a hot beverage of choice, acknowledge the Yin-Yang nature of reality and choose happiness.
No curve goes up or down for ever. I'm not a financial adviser.