Is it obligatory for me to complete my PhD studies to please my mother, who considers my unwillingness to do so an act of disobedience, knowing that this causes me severe psychological distress?

The general rule for obeying parents is the obligation to obey them in matters that benefit them and do not harm the child. You are not obliged to obey your mother in her desire for you to study a specific major. However, if this study does not harm you, then it is preferable to honor her by acceding to her wish. For honoring one's mother is among the greatest acts of nearness to Allah, and "I do not know of any deed closer to Allah - عز وجل - than honoring one's mother."