What is Ego?

…nobody speaks of the ego, because nobody knows it. It is such an intimate companion that one does not even recognise its existence; and yet so long as it is there one will never have the divine consciousness.
 The ego is what makes one conscious of being separate from others. If there were no ego, you would not perceive that you are a person separate from others. You would have the impression that you are a small part of a whole, a very small part of a very great whole.Onthe other hand, every one of you ismost certainly quite conscious of being a separate person.Well, it is the ego that gives you this impression. As long as you are conscious in this way, it means that you have an ego.
When you begin to be aware that everything is yourself, and that this is only a very small point in the midst of thousands and thousands of other points of the same person that you are everywhere, when you feel that you are yourself in everything and that there is no separation, then you know that you are on the way towards having no more ego.
[CWM2, 3:241]
…if you are sincere, you will notice a small black spot (a tiny wicked idea, a tiny false movement, a small arbitrary decision) and that’s the source of the uneasiness. You will notice then that the little black spot comes from the ego which is full of preferences; generally it does what it likes; the things it likes are called good and those it does not are called bad—this clouds your judgment. It is difficult to judge under these conditions. If you truly want to know, you must draw back a step and look, and you will know then that it is this small movement of the ego which is the cause of the uneasiness. You will see that it is a tiny thing curled back upon itself; you will have the impression of being in front of something hard which resists or is black. Then with patience, from the height of your consciousness, you must explain to this thing its mistake, and in the end it will disappear. I do not say that you will succeed all at once the very first day, but if you try sincerely, you will always end with success. And if you persevere, you will see that all of a sudden you are relieved of a mass of meanness and ugliness and obscurity which was preventing you from flowering in the light. It is those things which make you shrivel up, prevent you from widening yourself, opening out in a light where you have the impression of being very comfortable.
[CWM2, 4:88]
The ego seems to have been indispensable at one time for the formation of the individual consciousness, but with the ego were born all the obstacles, sufferings, difficulties, all that now appears to us as adverse and anti-divine forces. But these forces themselves were a necessity for attaining an inner purification and the liberation from ego. The ego is at once the result of their action and the cause of their prolongation. When the ego disappears, the adverse forces will also disappear, having no longer any reason for their existence in the world.
[CWM2, 3:218]

The Three Forms of Ego

The meaning of the word, ahankara, [ego] has become so distorted in our language that often a confusion arises when we try to explain the main principles of the Aryan Dharma. Pride is only a particular effect of the rajasic ego, yet this is the meaning generally attributed to the word ahankara; any talk of giving up ahankara brings to the mind the idea of giving up pride or the rajasic ego. In fact, any awareness of ‘I’ is ahankara. The awareness of  ‘I’ is created in the higher knowledge Self and in the play of the three principles of Nature, its three modes are revealed: the sattwic ego, the rajasic ego and the tamasic ego. The sattwic ego brings knowledge and happiness. ‘I am receiving knowledge, I am full of delight’— these feelings are actions of the sattwic ego. The ego of the sadhak, the devotee, the man of knowledge, the disinterested worker is the sattwic ego which brings knowledge and delight. The rajasic ego stands for action. ‘I am doing the work, I am winning, I am losing, I am making effort, the success in work is mine, the failure is mine, I am strong, I am fortunate, I am happy, I am unhappy’— all these feelings are predominantly rajasic, dynamic and generate desire. The tamasic ego is full of ignorance and inertia. ‘I am wretched, I am helpless, I am lazy, incapable and good for nothing, I have no hope, I am sinking into the lower nature, my only salvation is to sink into the lower nature’— all these feelings are predominantly tamasic and produce inertia and obscurity. Those afflicted with the tamasic ego have no pride though they have the ego in full measure but that ego has a downward movement and leads to death and extinction in the void of the Brahman. Just as pride has ego, in the same way humility also has ego; just as strength has ego, in the same way weakness also has ego. Those who have no pride because of their tamasic nature are mean, feeble and servile out of fear and despair. Tamasic humility, tamasic forgiveness, tamasic endurance have no value whatsoever and do not produce any good result. Blessed indeed is he who perceiving Narayana everywhere is humble, tolerant and full of forgiveness. Delivered from all these impulsions coming from the ego, one who has gone beyond the spell of the three modes of Nature has neither pride nor humility. Satisfied with whatever feeling is given to his instrumental being of life and mind by the universal Shakti of the Divine and free from all attachment, he enjoys invariable peace and felicity. The tamasic ego must be avoided in every way. To destroy it completely by awakening the rajasic ego with the help of knowledge coming from ‘sattwa’ is the first step towards progress. Growth of knowledge, faith and devotion are the means of liberating oneself from the grip of the rajasic ego. A person predominantly sattwic does not say, ‘I am happy’; he says, ‘Happiness is flowing in my heart’; he does not say, ‘I am wise’ he says ‘Knowledge is growing in me.’ He knows that this happiness and this knowledge do not belong to him but to the Mother of the Universe.
[Sri Aurobindo, Bengali Writings (Translation), p.179-80]

How to abolish the Ego? Why is it so difficult to get rid of it?

First of all, you must want to do it, and there are very few people who want to. And that is exactly what they say, it is this justification of their way of being, “That is the way I am made, I can’t do otherwise. And then, if I change this, if I change that or if I do without this thing or if I get rid of that other, I shall no longer exist!” And if one doesn’t say this openly, one thinks it. And all these little desires, these little satisfactions, these little reactions, all these small ways of being, one clings to them, clings hard – one sticks to them, one doesn’t want to let them go. I have seen hundreds of cases where someone’s difficulty had been removed (with a particular power a certain difficulty had been removed), but after a few days he brought it back with enthusiasm. He said, “But without that I do not exist any longer!” …You cannot imagine it, you don’t know how very difficult it is to separate oneself from this little ego; how much it gets into the way though it is so small. It takes up so much room while being so microscopic. It is very difficult. One pushes it away in certain very obvious things; for example, if there is something good and someone rushes forward to make sure of having it first, even jostling his neighbour (this happens very frequently in ordinary life), then here one becomes quite aware that this is not very, very elegant, so one begins to suppress these crudities, one makes a big effort – and one becomes highly self-satisfied: “I am not selfish, I give what is good to others, I don’t keep it for myself”, and one begins to get puffed up. And so one is filled with a moral egoism which is much worse than physical egoism, for it is conscious of its superiority. And then there are those who have left everything, given up everything, who have left their families, distributed their belongings, gone into solitude, who live an ascetic life, and who are terribly conscious of their superiority, who look down at poor humanity from the height of their spiritual grandeur – and they have, these people, such a formidable ego that unless it is broken into small bits, never, never will they see the Divine. So it is not such an easy task. It takes a lot of time. And I must tell you that even when the work is done, it must always be begun again.
[CWM2, 4:332-33]
The ego thinks of what it wants and has not. This is its constant preoccupation.
The soul is aware of what it is given and lives in endless gratitude.
All bitterness in life always comes from the ego refusing to abdicate.
Without the play of ego, there would be no conflicts; and if there were not in the vital a tendency to drama, there would be no dramatic happenings in life.
[CWM2, 14:257]
The extent of your difficulties gives you the measure of your ego.
[CWM2, 14:258]