Anger is considered one of the most destructive traits. Jacob strongly admonishes his children Shimon and Levi, "Accursed is their rage for it is intense, and their wrath for it is harsh." King Solomon warns, "Anger resides in the bosom of fools." The Midrash criticizes Moses for becoming angry: "Rabbi Eliezer states: in three instances Moses came to be angry and thus came to err: Upon being angry at Elazar and Itamar the sons of Aaron; after being angry with the commanding soldiers who returned from battle with Midian; and upon being angry at the Children of Israel when they demanded water."
It is puzzling that "anger" is so destructive and is nevertheless not the subject of a direct commandment. There is no mitzvah "Thou shalt not be angry." Rabbi Chaim Vital addresses this question, and answers it with a very profound concept. Before we ever get to the point of performing Torah precepts there is a need to develop our basic character. The traits that comprise our character determine the way in which we fulfill the Torah. We must spend our energy in perfecting these aspects of ourselves, and once these are properly developed we can perform any mitzvah with relative ease.
Conclusion: Overcoming anger is a foundation for the proper fulfillment of the entire Torah, and is therefore not counted as a separate mitzvah.