I don't think that's necessarily true.  Conceptions of identity might usually, often, develop over a period of years and decades but I don't think that this is necessarily the case.  Actual events which are momentous enough to alter broad publics' ideas of themselves can occur and produce influences in much shorter stretches of time.

  E.g.:  A war is lost or won, a currency collapses, a heroic breakthrough in a long-standing problem of domestic or international affairs, the restoration of an institution--one formerly thought definitively lost, the restoration of rights and freedoms which had either been suppressed or rendered de facto "dead letter", these can, with relative alacrity, alter notions of group identity by infusing or inspiring renewed faith, self-esteem, mutual good regard among differing groups previously disaffected, etc.

    An important part of what's been so badly degraded in society are these very aspects of faith, self-esteem, mutual good regard among differing groups previously disaffected.  When the political institutions whose respectable and dependable  operation underpins a people's belief in their political system's integrity are degraded, the consequences can be serious loss of faith, etc.  Conversely, the restoration of these same can contribute to a renewed or improved belief in one's political system and, by extension, a greater optimism, sense of self-esteem and improvements in things related to these.

   So, rather than exhorting people to "feel more optimistic," or to "have courage", "keep faith/hope alive", the practical course is to take effective actions which, when seen, broadly inspire these feelings without having to call for them.

   For these reasons, I believe it was a very serious and profound mistake for Obama to have, as he seems to have done, decided to "turn the page" on the Bush/Cheney years and the work of holding those in it responsible for their many and gross illegal acts, acts which dealt severe blows to the nation, to the public's sense of identity as a free, fair and just people, to the credibility of the conception of the United States as a democratic society.  It is not one, in fact.  But the Bush/Cheney tenure brutally made this fact manifest to many who had previously been able to sustain a belief in the myth of their society's democratic character.  To maintain such a faith today requires more than most people (other than the most die-hard conservatives) can muster in suspension of doubt or disbelief.

   If our identities and our myths have been made ineffective, that is large part because in the real waking world of daily life, actual events have revealed them to be, to have been for some time, even, empty of substance--and these revelations have come in ways that have been brutally shaking.

   When the WTC towers collapsed, the event in itself was not so much the world-changing event that many claimed.  Rather, it was a marker. For certain ways of belief, many held so unselfconsciously that they were hardly noted, the towers' collapse made the falsity of those beliefs suddenly and dramatically manifest and if they didn't fall with the towers, they fell in the hours, days, weeks and months which followed their collapse.  In fact, though, the processes long predate the events of that single day.

"In such an environment it is not surprising that the ills of technology should seem curable only through the application of more technology..." John W Aldridge