Chronic congestion and post-nasal drip?

For years I have suffered from allergy related congestion that never, ever clears up. I am constantly hoarse, constantly sniffling, and often experience sinus headaches and sore throat. By Dr.s recommendations, I have tried Benadryl, Zyrtec, Allegra, Claritin, and Drixoral, but none of them really work for me. Honestly, I'm totally used to this problem, but if there is a way to resolve it, I would be forever grateful.