Sky, I have a Benelli 12 gauge Nova Pump and the things stock is polymer. I hear you all too well because when I started shooting it the first word out of my mouth was, "WHEEEEEEW!". Next to me I had my father telling me I wasn't doing this, or that, when I knew for a fact I was. Tired of hearing his I handed it to him with almost grunt saying, "Here!". So he takes it so he can show me what's up and how to do this and he fires the trigger and says, "WHEEEEEEEW!".
Now when it came to getting over the kick, which I eventually did very succesfully, I started off shooting it without really focusing on my target as the recoil. I pressed forward using reload tactics, proper sighting, trigger control and stance while firing rounds consecutively. Eventually my body just got used to it and while the shotgun can still jostle someone I'm pretty comfortable with it and have no problem handling or shooting it anymore.