Actually there are many reasons to be ashamed of our history.

 One of the major reasons political, social and economic progress  is so slow and fraught with hostilities is because we are programmed to believe  our history is nothing but glorious and honourable, with only a few honest mistakes made by leaders living by standards of the time.

  Look at the US for example. Their descent into authoritarianism (fascism) is largely due to their having gloried in their violent history to such an enormous extent they took way too long to admit how wrong and shameful their historical icons were ( For example, slavery and genocide of the American Indians). Now they realize it, but don't like to admit it. If they admit it, they don't like to change it.
   Progress, based on what we realize is right today, takes so long, largely because we take too long to admit how wrong the historical leaders were. Our realization of what's right and wrong lags way behind the historical perspectives.