Is it permissible to remain with the current wife despite a lack of understanding, feelings of regret, and thoughts of a former beloved, or should one divorce and marry the former beloved? And would divorce in this situation be considered a sin?

Divorce, when needed due to a wife's bad character, is permissible, and the husband does not incur sin or wrong the wife by it. However, divorce should not occur until all means of reconciliation have been exhausted. If the spouses can continue to live together amicably, that is preferable. The husband is advised to strive to reform his wife and reach an understanding with her; otherwise, two arbiters from their families should mediate. There is no objection to marrying a former wife while having a current wife, provided the parents agree.