How to get rid of post nasal drip ?

I ride horses and have horrible allergies all year just from being outside and probably breathing in allergens and hay particles. The post nasal drip and cough are persistent. I use a humidifier all night, take Zyrtec before bed and Flonase. I try to sleep sitting up and right now I have to have multiple Cough drops to stop coughing while sitting in bed. I even started using a nettipot. My eyes get crusty and goopy too and it can be really hard to sleep at night. I’ve already been to the doctor twice and they just tell me to take the Zyrtec and Flonase. I don’t want to give up riding, is there anything I can do? This has gone on for most of the last two years.