It seems that you have had skin tests or blood tests for allergy that all came back as negative, and hence the diagnosis of non-allergic rhinitis
The treatment is long term use of nasal corticosteroids
plus or minus oral antihistamines (although of limited benefit). The nasal corticosteroid recommended is mometasone
furoate 1 spray per nostril once daily (start with twice daily for a month). You can also use Azelastine
spray 1-2 sprays three times a day for bad itching if required.
It is a difficult condition to treat completely and you are not alone.