The territory in the east already considered itself relatively un-Polish by 1945. In 1939, the eastern third of Poland was annexed into the Soviet Union as part of the Molotov-Ribbentrop Pact. The Russians proceeded to deport hundreds of thousands of Poles to Siberia, Kazakhstan, and elsewhere. These deportations, and tbe fear of them, largely shattered Polish identity in the east. There are certainly still Polish minorities in the far west of Ukraine and Belarus, but those communities are only shadows of their former selves.
Actually, the same process bolstered the Polish national identity in the newly annexed regions of Silesia and Pomerania (southwest and northwest Poland, respectively). In an attempt at vengeance for war crimes committed by the Nazis, Poland began deporting Germans living in what was now western Poland into Germany. Historians debate how much influence the Soviet Union, which was now the de facto overlord of Poland, had on the decision to deport German citizens. However, most non-Polish historians agree that the action was not justified and that the Polish were essentially committing war crimes of their own. Records taken from the West German government in 1950 suggest that over eight million ethnic Germans were deported or fled from Polish territory. The interesting thing about the deportation of Germans is that it often flies under the radar because we imagine the Soviets and Nazis as the 'bad guys' during the 40s. Regardless, it was a major factor in Pomerania and Silesia being accepted as part of the Polish nation, and being less equated with Germany.
Source - "The Struggles for Poland", Neal Ascherson
If anything, the transfer of territory and population strengthened Polish identity. The Soviet Union annexed a large part of what was prewar Poland, but the land annexed (the Kresy) wasn't very Polish. It was by far the least literate, least industrialized part of the country, and as a result there wasn't a particularly strong national identity at all among the people who lived there.
After World War II there was a huge movement of populations, especially in eastern Europe, in an effort to make nations more homogeneous and to prevent irredentist claims on the bases of national minorities (i.e., the Sudeten Germans). The largest population to be deported back into their new homeland was the Germans, but Poles made up a large number of these as well. The USSR deported Poles from its annexed territories into the newly recreated Polish state. This was because Poles tended to have a very strong national identity, but also served to remove much of the intelligentsia from Lviv and Vilnius. Some of the ethnic Baltic, Ukrainian, and Belarussian intelligentsia were also deported into Poland from the Kresy, and many of the poorer ethnic Poles who worked as farmers or laborers were denied the opportunity to resettle in Poland.
In short, the territory Poland lost wasn't exactly part of the Polish nation to begin with. About 30% of interwar Poland's population was not ethnically Polish and most of those non-Poles were either Germans, who were deported over the Oder-Neiße Border, or a mix of Belarussian, Ukrainian, or Baltic, who were absorbed into the Soviet Union, leaving the Polish state with a non-Polish population of something like 5% today.
That's not to say the culture in Poland wasn't affected by the population and territory transfers. Poland lost most of its farming territory and gained heavily industrialized, urban regions in Silesia and Pommerania from Germany. Throughout the 1950s and 1960s there was a great fear among the Poles who settled in the new territories of another invasion by Germany, because the West German government refused to recognize either the GDR or the GDR-Polish border, claiming that East Germany and the former German territories in Poland were just temporarily under Soviet occupation. These upheavals definitely affected Polish national identity- even to this day, every demographic map of Poland shows clearly the old borders, which are now called Poland "A" and Poland "B". But I don't think that there was a crisis of any Polish national identity needing to be "regained" as your question insinuates.
I'm not sure I understand the question. Doesn't "national" mean something along the lines of identity, as in "this group of people all speak the same language, have the same customs, wear the same clothes, etc"?
It's sort of tautological.