"I wish that the English remembered more of Irish history and the Irish remembered less of it." Irish novelist Elizabeth Bowen, 1899-1973
That could apply to many places. I think, however, that there is a nuance that the academics and almost-powerful people of a place remember more than the population as a whole, and it is they who remember too much. They are held aloft by those in other centers of power who remember too little, which gives them a reason for not just getting a real life and moving on. The English remember very little about Cromwell and the Restoration, and what they know is not only oversimplified, but wrong. Yet it doesn't much matter anymore, does it? People have families and jobs and football teams to follow.
What is the cutoff? I wonder. Living memory? When you have moved up enough in the world to be oppressing others yourself? As long as you are failing to thrive you will continue to remember, I suspect. One would think that in every generation a few would move up and forget all but the ethnic dishes (themselves often inauthentic, as with the Irish) and the colors of the flag, but in the Balkans 1381 is still alive in some memories.
It is true internally as well. Why on earth do the Flemish and Walloons have even a remote distinction in their minds, for example? How many centuries of being oppressed together and oppressive together does it take to see yourselves as one?