I had difficult in breathing when I was 6-7 mostly in congested areas. But, it never happened in later year. How did it cure itself.?

Airway grew. At age 6 your airway is not at its fullest diameter and any cold or allergy would narrow the airway to a greater degree than in adults. As the airway grows the same exposure causes lass narrowing and symptoms of breathing difficulty.
Not enough info. There are many possible answers to this clinical question. Tonsils and adenoids achieve their largest size at age 4-6. With growth and development, the airway increases relative to the tonsil size and obstruction may be relieved. This is only one of many possible reasons.