Since antihistamines and decongestants haven't relieved your symptoms, the medication may actually be drying out your nasal passages, making the mucus stickier. To relieve this, I recommend discontinuing those medications and trying alternatives to thin the secretions. You can use a saline nasal spray or rinse to flush and moisturize your nasal passages. A steroid nasal spray, such as fluticasone, can help reduce inflammation, while an oral mucolytic like guaifenesin can help thin the thick mucus. Regular steam inhalation and staying well-hydrated will also provide relief. If your symptoms persist, please consult a doctor to rule out a sinus infection.