Is forgiveness and expiation available for a woman who sought divorce and absolved her husband of her rights, then felt guilty after the divorce?

If the situation is as you described, then your request for divorce (talaq (definition)) or khul' was not a sin requiring repentance. This is because it is permissible for a woman to seek khul' if she dislikes her husband due to his character, physical appearance, religiosity, or other reasons, and she fears that she will not be able to fulfill God's rights in obeying him. This is based on the Almighty's saying: "Then if you fear that they will not uphold the limits of Allah, there is no blame upon either of them concerning that by which she ransoms herself." The prohibition against seeking divorce applies when there is no valid reason, as the Prophet, peace and blessings be upon him, said: "Any woman who asks her husband for divorce without any strong reason, the scent of Paradise will be forbidden to her." In all cases, a Muslim should consistently seek forgiveness, repent, and strive in righteous deeds.