Is a young man considered to have wronged a girl by his repeated hesitation and indecision about marrying her, or by leaving her after she has become attached to him?

You must stop going out with, touching, and speaking to foreign women unnecessarily, as this is not permissible in Islamic law. It does not appear that you have wronged the girl; rather, if you had proposed to her, you would have had the right to break off the engagement, because an engagement is merely a promise of marriage. You should have defined your position regarding marrying her and abandoned indecisiveness and playing with her feelings. A successful marital life does not require love between the two parties; rather, good choice and seeking Allah's guidance (istikharah) are what matter.