A DNS(deviated nasal septum) does pre-dispose one to recurrent infections on the affected side.
Common Cold is a viral infection and is generally afflicting adults and children during change of seasons. A purely viral cold is self limiting and would generally last 3 to 4 days and is often treated with simple decongestants and antipyretics.
However in patients with Allergies or Blockage in the Sinonasal system (DNS, Polyps) increased mucus secretions are not able to flow out and stagnation of secretion happens making them thick sticky and further bacterial infection occurs.
This explains the constant build-up behind your nose.
You should undergo Diagnostic Nasal Endoscopy to assess the site and severity of blockage and also Allergy Testing if appropriate to seek out long term remedy.
Frequent bouts of sneezing with watery nasal discharge and associated itching in the nose, palate and throat can be a manifestation of Nasal Allergy. It may be associated with inflammatory nasal obstruction due to swelling of the turbinates and nasal mucosa. It is generally provoked by exposure to inhalant allergens such as dust, smoke, pollens, perfumes, cold air etc.
Nasal Allergy should be evaluated by Nasal Video Endoscopy to document the severity and extent of allergic changes including polyp formation. An Allergy Test by In Vitro testing should also be done to establish allergens.
Avoidance of allergens is of great help along with anti allergics. Nasal Allergy is best treated using Local Antiallergic and Steroidal Antiinflammatory sprays which can be used intermittently or continuously depending on the allergy pattern.
You could discuss log-term treatment options with an ENT specialist.