There are multiple causes of chronic nasal discharge / rhinitis / sinusitis, including allergies, foreign bodies, bacterial / viral / fungal infections, parasites, autoimmune diseases, and cancer to name a few. Given that this has gone on for several years as you've stated I don't think a simple solution likely exists (e.g. such as benadryl or another OTC medication). Coughing is concerning for something lower down in the respiratory tract, such as bronchitis like you indicated. Older age does raise the concern for nasal cancer in dogs with chronic nasal discharge. These are commonly difficult cases to treat if they don't respond to outpatient therapy such as antihistamines, steroids, antibiotics, etc. Diagnostics that can be pursued (and often are necessary in order to actually achieve a definitive diagnosis and thus definitive treatment recommendations) include routine labwork (bloodwork, urinalysis, fecal parasite check), radiographs (x-rays) as a rather poor way of imaging to screen for asymmetries and indications of aggression, nasal scope to visualize and obtain biopsy samples for histopathology and culture (bacterial, fungal) and look for evidence of parasites (nasal mites can be seen with scoping sometimes), and advanced imaging such as CT or MRI. Initial diagnostics are often performed with a local veterinarian, with some initial treatment trials, it sounds like you've gone through this already. If this were my case, I might suggest repeating x-rays as well as the next step, given the chronicity, to assess for changes. At that same time, her nasal sinus could be flush with saline. I've had this resolve some difficult cases, but more commonly it allows for temporary relief only. Ultimately, often times referral to a specialist that can pursue rhinoscopy for biopsy / culture and advanced imaging to look at the nasal passages much better than x-rays can is often necessary, and otherwise treatments are just educated guesses towards what a cause might be (again, e.g. antihistamines, steroids, antibiotics, antifungals, antiparasitics etc.). For brief relief, it is ok to administer a couple drops of Little Remedies (or similar brand) phenylephrine drops into either nostril, every 6-8 hours. Humidifiers may provide some relief as well. Chronic antihistamine administration (benadryl) is very safe to continue as well.