What should a young man do when his mother rejects the girl he has chosen to propose to, despite his insistence and attempts to convince her?

Your mother is the most deserving of your obedience and devotion, and you must strive to comfort her and seek her pleasure. Try to gently and kindly persuade her to agree to this girl, who possesses good character and religious commitment. You can find a replacement for a fiancée or a wife, but you will never find a replacement for your mother.