Is it permissible for a Muslim man to tell a married woman he loves her?

You have gravely erred by writing to this woman, expressing your love for her, due to the potential consequences of corrupting her relationship with her husband, which Islamic law terms as "takhbib" (incitement/corruption). If Allah instills the love of a woman in a man's heart, he is obligated to preserve his chastity from engaging with her in anything forbidden, for the inclination of the heart is a type of zina (fornication/adultery). Therefore, you must repent to Allah and completely turn your heart away from her, especially since she is married.