(This post looks at the the meaning of the word myth in the study of history. It doesn't deny that the primary meaning of the word is 'a legendary tale', 'a sacred story concerning the origins of the world and the creatures in it'. And sorry about the length.)
A myth is a set of political and cultural beliefs and messages derived from a common or shared historical experience. I think this is what most historians would mean if they used the word, certainly those who study the practice of history. The national myth is simply one of the most common.
A national myth may be mythical in the sense of being false, but it may be mythical in other senses as well. It might simply over-dramatize true incidents, omit important historical details, or add details for which there is no evidence... National myths serve many social and political purposes. In totalitarian dictatorships, national myths often exist only for the most shallow purposes of state-sponsored propaganda... But national myths usually also exist in more liberal regimes, serving the purpose of inspiring civic virtue and self-sacrifice, or shoring up the power of dominant groups and legitimating their rule. (link)
For example, there are the Crusader myths; that the Crusades were a series of glorious wars of religion, fought by chivalric, pious knights, rather than squalid occasions of murder, theft, rape and betrayal in which the Crusaders were as often at each other's throats as fighting to free the Holy Land.
Bernard of Clairveaux provides the clearest expression of this myth:
They do not go into battle stormily and without thought, but with due consideration and caution, peaceful like true children of Israel... they are gentler than lambs and more ferocious than lions so that one has doubts whether to call them monks or knights. Yet they deserve both names, because they partake in the gentleness of monks and the bravery of knights.(link)
Bernard was to be disappointed, and later ascribed the failure to the worldly ambitions and vanity of princes.
There are many more: America has the myths of the Old West, stories designed to misdirect from brutal historical truths, and emphasise the messages the culture wishes to associate with the event. So the conquest of the west is cast as expansion driven by the finest qualities of the human spirit, rather than genocide and theft driven by greed and eastern capitalist expansion.