"IQ indicates a greater capacity for complex cognition for problems new to you. But what we apply that capability to is another question. Think of our minds as searchlights. IQ measures the brightness of the searchlight, but where we point it also matters. Some people don't point their searchlights at the other side of the case much, for many reasons - entrenched ideas, avoidance of what might be disturbing, simple haste. A higher wattage searchlight in itself is no protection against such follies."
So basically, how well you can think (IQ) is not nearly as important as what you choose to think about (rationality). If you cannot learn to escape cognitive biases and analyze life properly, a high IQ won't do much good.
The good news is that it is fairly easy to learn about and defeat these cognitive biases. The article mentions several. The main thing is to get in the habit of questioning your own thoughts and conclusions. Always ask yourself 'Why am I thinking this?' or 'Why do I believe that?'. Learn to recognize when you are doing something because of hasty intuition, emotions, authority, or social conformity.