What is the truth behind my fiancée's sudden change toward me, and what should I do after some fortune-tellers told me she is bewitched?

Hearts are in the hand of Allah; He turns them as He wills. The preservation of blessings is through obedience to Him, while sins are a cause of calamities and the loss of blessings. Your relationship with this girl is forbidden because she is a non-mahram to you, even if she is your fiancée. Your going to soothsayers and asking them is a grave sin and a major one, as the Prophet, peace and blessings be upon him, said: "Whoever comes to a soothsayer or a diviner and believes what he says has disbelieved in what was revealed to Muhammad." And his saying: "Whoever comes to a diviner and asks him about something, his prayer will not be accepted for forty nights." We advise you to complete your marriage to her, and if that is not possible, then seek another woman of religion and good character.