Try salt water irrigation. I used to be congested a lot from allergies until I started rinsing my sinuses and nasal passages with salt water.
You can use a Neti pot which looks like a miniature aladin's lamp, bulb syringes, or large syringes made for the purpose. The syringes do a better job but the Neti pot works for me and it is easier and more comfortable.
I found that using salt water to rinse out the snot, mucus, and allergens is far more effecitve than any medication I tried.
did you try ear,throat,nose specialist?perhaps you have a diviated septum?