Is a person who is afraid of leading the prayer sinful for not coming forward to lead it, even though he has memorized a large portion of the Qur'an?

If there is someone in the congregation who has memorized Al-Fatihah and recites it well, there is no sin upon you for not stepping forward to lead the prayer, even if your recitation is better than his. His imamate is valid, and the communal obligation (fard al-kifayah) is fulfilled by it.

However, if there is no one among them who has memorized Al-Fatihah, or if there is a defect in their recitation that alters the meaning, then leading the prayer becomes incumbent upon you, and you must step forward and overcome your fears.

If these fears affect the pillars of prayer, do not step forward as long as this is your condition. However, do not surrender to these fears; try to overcome them by contemplating the greatness of Allah and pondering the Qur'an.