There are many causes of mouth breathing. Few common causes are chronic allergies, tonsil hypertrophy, nasal polyps, deviated nasal septum, constricted upper airways, adenoids etc.You will need a complete Ear nose throat examination by an ENT specialist. A dental check up will also be helpful. A physical examination is very important for determining the correct cause of your symptoms. Treatment will depend on the underlying cause. I hope it helps.
Best wishes and kind regards!