meaning - Is there a word to describe an individual who has complete control over his negative and positive emotions?

Context can easily change the word that will answer the OP - is the person saintly or pyschopathic, a powder keg of buttoned up frustration and anger, or averagely content and sociable?

Someone who is able to experience positive and negative emotions without either self-harming or acting sociopathically might be said to have balanced their emotions, in the sense that neither good nor bad emotions disrupt their ability to maintain an 'even keel' and go about their daily business.

Alternatively, you could say that this same person is self-aware, unreactive, in good mental health.