Is a man's love for a woman other than his wife, or merely thinking about her, or supplicating to marry her, or wishing for his wife's separation or death so that he may marry another, considered forbidden? And how can he repent and be saved from what he feels?

This type of love is forbidden by religious law due to the heart being occupied by a woman in an unlawful manner. There is no objection to marrying the girl if she is religious and of good character, and you are able to treat her and your current wife justly. It is permissible to pray for the facilitation of marriage to her. Before that, you must completely cut off contact with the girl and be careful not to neglect the rights of your wife, especially if she is excellent. Remember the words of Allah Almighty: (And it may be that you dislike a thing while it is good for you), and the words of the Prophet, peace and blessings be upon him: (A believing man should not hate a believing woman; if he dislikes one of her characteristics, he will be pleased with another), and (None of you truly believes until he loves for his brother what he loves for himself). It is not permissible to wish for the death of one's wife.