How can marriage be mended when the choice of partner is unsuitable, and is divorce the first solution?

If the wife is religious and of good character, then keep her and treat her well, for not all homes are built on love, but rather on cohabitation with kindness, as Allah Almighty says: "And live with them in kindness. For if you dislike them - perhaps you dislike a thing and Allah makes therein much good." (An-Nisa: 19). If she is neglectful in her religion or ill-mannered, then be patient with her and try to reform her through advice and guidance. If she does not respond, then consider divorcing her, but do not rush into it unless it becomes clear to you that it is the most suitable option.