The sin of pride goes well beyond merely feeling inordinately good about one’s self, or one’s accomplishments.
The sin of pride is a sin of the will, not a sin of feelings. It is the sin one commits when one places one’s will above the will of God. It is not merely failing to conform one’s will to the will of God – everyone does that; all fall short. This sin is committed when one knows what God’s will is, but chooses to act at the direction of one’s own will where it differs from the will of God.
Many who misunderstand the teachings of the Church think that the greatest possible sins are sexual in nature. They are not. Pride is the sin that led to the fall of mankind – a direct act of will contrary to the expressed and know will of God. Mind you, in this society, the confusion regarding sexually based sins (which can indeed be serious, for sure) is understandable, since society utterly disregards the will of God in matters of sexual morality, and encourages people to do the same.
It is sometimes useful to consider the opposite of pride, humility, to get some insight. Humility has nothing to do with smart people saying they are dumb, or courageous people saying they are cowards. Consider a craftsman - say, a cathedral builder. He comes upon a beautiful cathedral he built, and one another craftsman built, and he is equally appreciates and proclaims the beauty of both. That’s humility. And it is not pride when he is appropriately moved by the beauty of the one he built.
In his writings, C.S. Lewis made very good arguments for considering pride to be the “master sin.”