I believe that the EU was the best thing that could've happened to Poland.
key word is `was`. Yes, at one historical moment EU was best solution for Poland. There was common interests between Poland and western Europe that Poland join the EU and that`s it, times and situation changed.
We all facing dramatic changes in global arena. This world isn`t uni-polar but rather multi-polar now. Money flow in the world shift on the behalf of East. Then, today`s Poland facing reality of rapid assimilation, ethnic weakening of Poles, depopulation from one side and reality of new settlers coming to Poland, without control of Poles or institutions of Poland.
There are many countries, on Baltic-Balkan line, who are threatened by challenges similarly to Poland. Within that group of countries Poland would seek solution for its problems, not within EU and not with western Europeans who have quite different problems and desires then Poland.
So, nobody sane cam`t blame Poles for rising of anti-EU feelings in Poland.