For my sins, Poland has been (and possibly always will be) an Eastern European country. Simply because, from my perspective it's "over there", on the fringes, beyond etc. Being a neighbour of obviously Eastern European countries helps.
As I have said, if I thought for a minute that this term was offensive to Polish people I would never use it.
I think that you are getting a bit confused by trying to tie up the geographical concept with the political one. Geographically speaking , although it is not anymore, it used to be an East European country. This was the result of Poles controlling large swathes of eastern land-namely the Ukraine and Belarus, countries which were in effect colonies in all but name, were the gentry and the Ruling classes were Polish and the peasantry was predominantly Ruthenian
Politically speaking it is more "western" than some of the countries in the "west", That's why in practiced early forms of democracy and rejected the divine right of kings centuries before some countries in the west. And that is why historically it sometimes defined its mission as bringing Catholicism into eastern Orthodox lands, this is why you now have a third religion called the Unites. In my opinion if you don't understand these two concepts, then you don't really understand Polish history.