For the last few years, one of my nostrils is always blocked, but they switch back and forth. it's not always the same one. how can i cure it?

Nasal Saline: What you are describing is classic nasal congestion, usually allergic. Everyone has a "dominant" nostril, and this switches back and forth every 4-6 hours. The other nostril is typically not completely blocked, but is a little tigher. If you have baseline allergies, this can be a little more bothersome. Sometimes nasal steroids (flonase, etc) are needed, but try saline spray first.

Normal nostrils: The nose has bony ridges inside each nostril called turbinates that swell as a result of normal blood flow patterns. Turbinates help humidify the air you breath. The swelling alternates from left to right on a normal cycle (every few hours typically). It usually does not require treatment unless you also suffer from allergies or chronic sinus problems.

This may be normal: Everyone experiences the "nasal cycle" in which structures inside the nose called "turbinates" alternately congest and decongest. This usually occurs every 3 to 4 hours. Usually, as long as one side is open it does not present a problem. I do have patients who become fixated on it and it drives them to distraction. In these patients i try steroid sprays, decongestants and even turbinate surgery.

Find the cause first: Very common complaint due selling of turbinates it may be normal or not.Allergy could give similar feeling , nasal polyp is another.Since it is been for a year you need to have nose exam. If no abnormal finding nasal smear could give valuable clue. Steroid or saline spray could of value.