I do not think you need to forgive, it is kind of an unforgivable action, not leaving you but the way in which he did and there's nothing to forgive in you; you do not want to forgive that you hurt and refused to see the worst in this person? It sounds like he is destined to be forever immature (did not develop well, not something he can "fix") and I would feel a little sorry for him that he cannot ever experience true love and contentment? Think of it like a heartbeat, you want those ups and downs, pains and joys and all he has as a fake is basically a flat line?