Talking Parsha - Nitzavim: Why is the easiest mitzvah so hard?

In one of his last speeches to the Jewish People, Moshe refers to 'this mitzvah.' Later he describes the opposite attitude in one of his last speeches to the Jewish People, Moshe refers to 'this mitzvah.' What mitzvah is he referring to? And why does he later describe the opposite attitude? What happened to the nation of Israel?