Q. 1. Does the Catechism of the Catholic Church teach us anything about how a Catholic can be forgiven of the sin of abortion?
A. 1. The following two sections of the Catechism of the Catholic Church state:
# 2272 Formal cooperation in an abortion constitutes a grave offense. The Church attaches the canonical penalty of excommunication to this crime against human life. "A person who procures a completed abortion incurs excommunication latae sententiae," [CIC, can. 1398] "by the very commission of the offense," [CIC, can. 1314] and subject to the conditions provided by Canon Law. [Cf. CIC, cann. 1323-1324] The Church does not thereby intend to restrict the scope of mercy. Rather, she makes clear the gravity of the crime committed, the irreparable harm done to the innocent who is put to death, as well as to the parents and the whole of society. 
# 1463 Certain particularly grave sins incur excommunication, the most severe ecclesiastical penalty, which impedes the reception of the sacraments and the exercise of certain ecclesiastical acts, and for which absolution consequently cannot be granted, according to canon law, except by the Pope, the bishop of the place or priests authorized by them. [Cf. CIC, cann. 1331; 1354-1357; CCEO, can. 1431; 1434; 1420] In danger of death any priest, even if deprived of faculties for hearing confessions, can absolve from every sin and excommunication. [Cf. CIC, can. 976; CCEO, can. 725] 
Q. 2. What does that mean in simple English?
A. 2. It means that anyone who has an abortion, participates in an abortion, or supports an abortion, he or she, by the previously mentioned actions, is automatically excommunicated from the Catholic Church. (C.C.C. # 2273)
Accordingly, that person can no longer receive any of the Sacraments of the Catholic Church, be it the Sacrament of the Holy Eucharist (Communion), Marriage or the Sacrament of Confession. For an excommunication to be lifted (ended), the sinner must approach the local Bishop, inform him of his/her "latae sententiae" excommunication through the sin of abortion and ask the Bishop to have the excommunication lifted (ended) and to be absolved of the sin of abortion. The Bishop may authorize a priest to specifically deal with this absolution on his behalf. (C.C.C. # 1463) Contrary to the popular belief of Catholics, this does not mean that all priests can absolve a penitent of the excommunication. Only a priest who is specifically appointed by the Bishop to do so, may do so.
Q. 3. Does that mean that if a woman had an abortion and confessed it to a priest who had not been specifically appointed by the local Bishop to lift her excommunication, that her sin was never forgiven?
A. 3. That is correct! There have been many instances where priests did not realize that an abortion incurs an automatic excommunication. As such, while they may enjoy the authority to forgive sins, they do not have the authority to lift excommunications. Consequently, while a person is excommunicated, that person cannot validly receive any of the Sacraments, that including the Sacrament of Confession. (Exception: In danger of death any priest, even if deprived of faculties for hearing confessions, any priest can absolve from every sin and excommunication.)
Q. 4. That being the case, most likely, a very large number of women never had their sin of abortion forgiven because they never had their excommunication lifted by their local Bishop or by a priest appointed by the local Bishop to do so.
A. 4. I agree with that. Most likely, less than 25% of Catholics who had an abortion, participated in one, or supported an abortion, actually had their sins forgiven because they did not observe the Church policy on the matter.
Q. 5. Would the same apply to Catholic politicians who support abortions?
A. 5. Yes! They are automatically excommunicated and are no longer able to enjoy any of the Church Sacraments. To receive any of the Church Sacraments after having been excommunicated, these politicians are drawing the wrath of God against them for continuing to insult the Lord Jesus after having shown little respect for life through the support of abortions.