mas·tery 
/ˈmæstəri, ˈmɑ:stəri/ noun
/ˈmæstəri, ˈmɑ:stəri/ noun 1 : knowledge and skill that allows you to do, use, or understand something very well : command — usually + of [noncount] ▪ We were impressed by her mastery of the subject. [singular] ▪ She achieved a complete mastery of French. 2 [noncount] : complete control of something
▪ He struggled to gain mastery of/over his fears.
▪ He struggled to gain mastery of/over his fears.
