To repent of the sin of abortion, there is the Sacrament of Repentance (Confession), at which the priest privately assigns penance in accordance with the spiritual and physical condition of the penitent, age, health and other circumstances. Abortion is one of the most serious sins. The Holy Church, according to the canons (rules), excommunicated those who committed the sin of abortion from communion for 10 years, on a par with murderers. In established church practice, this rule is not strictly applied, but the priest can assign a prayer rule to correct the penitent, so that he comes to true repentance.
In 2016, the Holy Synod approved the text of the prayer sequence with the canon of repentance about the sin of killing children in the womb (abortion).
In 2021, the Holy Synod approved the text of the canon to the Most Holy Theotokos, read by the woman who killed her child in her womb.
As repentant prayers, you can also use the canon of repentance to our Lord Jesus Christ or the Service to the Bethlehem Babies, written by St. Andrew of Crete.
St. Nicholas of Serbia in such a situation advises to do works of mercy. By works of mercy, first of all, we can understand active repentance to turn away our neighbors from this sin, that is, it is necessary to dissuade women from infanticide (abortion), help needy pregnant women, and participate in the activities of Orthodox defenders of life (pro-life).
You should not set your own prayer rule. And also use texts of prayers that are dubious from the point of view of church doctrine, for example, the akathist “to the repentance of wives who destroyed babies in their wombs.” Or the “Rule” of schemanun Antonia.
More details about the posthumous fate of unbaptized babies killed in the womb of their mother are described in the book of Archimandrite Raphael (Karelin) “How to return lost joy to the family.”