Ego is not the same as self-confidence or just 'confidence'.
To diminish one's ego
means to diminish all the problems that arise from having related to "this ego" in the first place.
Problems such as pride,conceit, self-centeredness,low self esteem, bruising one's ego" even fear,worry and anxiety.
All these things are seen from the perspective of an ego.The ego feels threatened,feels entitled ,feels hurt etc,and so forth.
Without the ego all this would seem very minor.
Which is a good thing because usually people are enslaved by it and imprisoned by it's wishes.
Therefore if you are free from these ego related problems then what you get is freedom and in freedom you will have no trouble being confident.And this sort of confidence is the best sort of confidence,
one that is free from greed,hate and delusion.