What is the ruling on marrying a righteous young man of good character and religious commitment, given the fear of his mother's harshness and temper, and the resulting psychological suffering?

Marriage is a source of tranquility, reassurance, and chastity for the soul, and the Prophet, peace and blessings be upon him, encouraged it. If the young man is religious and of good character, and you can avoid what provokes his mother's harshness and irritability, then agree to the engagement. Perhaps Allah will place affection and mercy between you. However, if you see that the mother's harshness and irritability are beyond your endurance and that happiness will turn into misery with her, then step away now. You are not obligated to live with her, for a chaste woman has the right to refuse to live with her husband's relatives, even his parents, due to the harm involved. And you must perform Istikhara (prayer for guidance), for whoever seeks guidance from Allah will never be disappointed.