Totasl immersion! Arrange breaks to get away from, it all for several days, a week, month, whatever, where nary an English wrod can ge heard. During such breaks plunge into the hinterlands and stay clear of Anglophone tourist traps.
As Polonius said, total immersion. Go to a village area where only Polish is spoken. Oh, and say a fervent prayer that they don't tell u to get lost in no uncertain terms, LOL. Villagers can be ferocious nationalists.