Could the depression and obsessive thoughts a girl experiences towards her fiancé, despite her and his repentance, and their preparations for marriage, be caused by magic or envy, and has the result of the Istikhara prayer not yet appeared?

If the young man is religious and of good character, then accept him as a husband, based on the Prophet’s (peace be upon him) saying: "If someone whose religiosity and character you approve of proposes to you, then marry him. If you do not do so, there will be tribulation on earth and widespread corruption." Make sure to continue performing Istikhara, as the feelings you are experiencing might be mere obsessions, or from Satan, or due to magic and the evil eye. In such a case, ruqyah (Islamic exorcism/healing) is necessary. If you still do not feel comfortable, then there is no harm in breaking off the engagement, as it is not binding, and either party may dissolve it unless there is no valid excuse.