Is what the girl did—rejecting the young man on the grounds of not wanting to be committed for several years until she completes her higher education and work—considered haram and hurtful to the young man?

What you did is absolutely correct by repenting from the relationship. What the young man mentioned is a beautification from Satan for the continuation of disobedience. So, do not be deceived by his words and do not go back to talking with him. Do not be sad about cutting off this relationship, as it is a religious obligation that pleases Allah. If you are pleased with him as a husband, then let him approach houses through their doors, let him propose to you through your guardian, and let him hasten the marriage contract.