Would I be unjust to the girl if I did not go back to her, even though my mother asks me to do so, and she is psychologically exhausted after the dissolution of the engagement, which was full of problems?

There is no blame upon you for leaving the girl and searching for another. You are not considered to have wronged her, and you are not obligated to obey your mother by returning to propose to her. Nor will you be disobedient to her by disagreeing with her on this matter, for parents do not have the right to compel their child to marry someone they do not desire. If the child refuses, they are not considered disobedient. However, if the girl is righteous, it is preferable to return to her.