How can one exit a forbidden love relationship that developed between a committed young man and his paternal aunt's daughter, knowing that this relationship negatively affects his faith and worship?

Your role as one who enjoins good and forbids evil makes you more responsible than others. It is not befitting for you to practice something you know is forbidden. A severe warning has been issued against those who enjoin good but do not practice it, and forbid evil but commit it. If you love a girl and she loves you, what prevents you from marrying her? The Prophet, peace and blessings be upon him, said: "Nothing like marriage is seen for two who love each other." If the impediment is that she is your brother's stepdaughter, then know that what is forbidden to you is your brother's daughter, not his stepdaughter, unless she was nursed by your brother or by someone who would make her your sister. Scholars have differed regarding the ruling on a wife nursing from the milk of her previous husband after marrying another. The Hanafis, Shafi'is, and Hanbalis held that the milk belongs to the first husband, while the Malikis believe that the second husband becomes a partner in the milk. However, if the girl was not of nursing age when your brother married her mother, then no prohibition occurs by the consensus of scholars. If marriage is not possible, then it is obligatory to stay away from her, because the trial of women is the most harmful of trials.