What is Ego ( Ahamkara ) and why is it difficult to control it ?

Ego is the self- arrogating aspect of the mind .It is the ego which separates the individual from unity with others and within himself , for the ego asserts “I-ness.” Ahamkara is the greatest obstacle to tranquility , for it is that which occupies the mind with whether we are better or worse , possess more or less , and have greater or lesser power than others .It is attended by desire , pride , anger , delusion , greed , jealousy , lust and hatred .The ego is the most difficult aspect of the mind to control , for its nature is such that it deludes even while one is striving to overcome it .It is that very part of the being that would not be controlled .

Swami Vishnu devananda from the book "Meditation & Mantras" pg .3 & 4