This tearful, cleansing lamentation comes from the heart of a believing Orthodox person.  With hope, she tenaciously clings to her Saviour with the boldness of one who knows the ultimate greatness of Christ's forgiveness.  Not attempting to hide from Christ's gaze the magnitude of her sinful actions, the writer of this akathist fully presents her fallenness to the Lord without leaving unspoken even a trace for which the demons might accuse her at the Last Judgment.  And out of the fullness of her repentance, in this liturgical expression of sorrow, she can freely call out:

O Lord, my Lord, my Joy, have mercy on me who am fallen.

What is an Akathist?  An akathist (Greek, akathistos) is a hymn dedicated to a saint, holy event, or one of the persons of the Holy Trinity. The word akathist itself means "not sitting." (Source:  https://orthodoxwiki.org/Akathist)