I believe this quote is very true.
Intelligent people are full of doubts because they look at things from several perspectives. This makes the situation they are dealing with more complicated and sophisticated. This lowers their courage and condfidence levels
On the other hand, stupid people see thinhs from one side only. They are ignorant about the other sides or points, thus, more courageous and confident.
The balance can be restored. Intelligent people need to work more on themselves, as intelligence is not enough sometimes.
Work on yourself! Believe in your capabilities, no matter what the world tells you.