We all have an ego. And your ego has the ability to deal with reality and stress differently than my ego, your mother's ego, or anyone else's ego. To that extent your ego is able to do all of this, maintain emotional stability, and deal with stress is your "ego strength". In a clinical setting (when dealing with psychological disorders) we can say that ego-strength is a person's capacity to maintain his/her own identity despite psychological pain, distress, turmoil and conflict between internal forces as well as the demands of reality.
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