When we read a book. We make abstractions of ideas and concepts. Make a whole picture out of it. And after a time say I can answer questions in this subject. Making abstractions and deriving concept requires time.
When we try to understand ourselves. We follow the same regime. To understand ourselves is to understand what we are in the now. In this moment. The moment we try to understand ourselves through abstractions. We become something else and next moment arrives. And we are still holding onto the abstraction of previous moment. Therefore we can say that understanding which is looking at ourselves isn't possible through abstractions or thinking. Or we can say that to read the book we need time. But we can't read ourselves through time which is abstractions or thinking.
Can we say that we can not have self knowledge? And who are these people telling you who you are !! When you yourself can never have the knowledge of the self ...