Is it permissible for a woman who has previously intended to divorce her current husband in order to return to her first husband, whom she loves, to ask him for a divorce, knowing that she has not disclosed this intention to her current husband? And does her unhappiness with him justify her seeking a separation?

If a man divorces his wife with three pronouncements, she is not lawful for him until she marries another husband through a marriage of desire, not for the purpose of making her lawful for the first husband. This is based on the verse: (And if he has divorced her [for the third time], then she is not lawful to him afterward until she marries another husband) (Al-Baqarah/230). The Prophet (peace and blessings be upon him) cursed the one who makes a woman lawful [for her previous husband] and the one for whom she is made lawful.

As for the divorced woman who marries a second husband with the intention of returning to the first, there is a difference of opinion among the jurists on this matter.

You are now a Muslim wife. It is not permissible for you to seek divorce (talaq) or annulment (khul‘) except for a legitimate reason, based on the Prophet’s (peace and blessings be upon him) saying: (Any woman who asks her husband for a divorce without any compelling reason, the scent of Paradise will be forbidden to her.) The meaning of "compelling reason" (al-ba's) is a necessity that necessitates divorce, such as the husband's ill-treatment or his immorality.

Women who seek annulment without a legitimate Sharia reason have been described as hypocrites. Among the reasons that permit seeking annulment are: the wife's dislike for her husband and her inability to give him his rights, or her fear of not being able to do so. This includes disliking his character, or his physical appearance, or his lack of religious commitment, or his withholding of her obligatory rights, or his failure to provide for her chastity.

If any of these reasons exist, it is permissible for you to seek annulment by returning the dowry. Otherwise, it is forbidden for you to do so, and patience is obligatory for you. Perhaps what you are experiencing is good, as Allah Almighty says: (But perhaps you hate a thing and it is good for you) (Al-Baqarah/216).