Throat really sore swnallowing left side couple months now. Otherwise well. Causes and treatment please?

Get medical check up. Sore throat usually means infection of the Tonsils/throat, upper airway, often caused by viruses. Sometimes it is more serious due to a bacteria called Strept throat and may require antibiotics before it will go away as most viral infections do go away in 2-3 weeks.
It is best to show your throat to a physician or ENT specialist so that further evaluation/ diagnosis be done, and treatment offered.