Why is the text of confession arranged according to the alef-beit (“Ashamnu, bagadnu,” etc.)? The reason is that when a person sins, the sin creates a kelipah with a body and a soul.

The soul is created from the desire for the sin and the body from its actual performance. If the sin is in thought or speech, the body of the kelipah is created from the letters.

When a person repents, the regret and the uprooting of the will nullifies the soul of the kelipah; and confession with words nullifies the body of the kelipah (for the lips’ movement is considered action).

Therefore we confess by mentioning the 22 letters of the alef-beit, in order to nullifiy the body of the kelipah formed from the 22 letters of foreign thoughts and sinful fancies.

(Derech Mitzvotecha, p. 76)