Why hasn't Allah answered my prayer despite my strong conviction, and why hasn't He granted me victory over those who wronged me? And how can I regain my trust and conviction in Allah after this despair?

What you are suffering from, namely obsessive thoughts and a negative opinion of Allah, is from the wiles of Satan. Its cure lies in supplication and in turning away from it. Do not preoccupy yourself with supplicating against those who have wronged you, for forgiving the oppressor is a beautiful trait. The non-fulfillment of a supplicant's request does not mean that the supplication has not been accepted. A servant is considered to be hasty if he says, "I supplicated but was not answered," thus despairing and abandoning supplication. There is no text that states that reciting Surat Al-Baqarah is a cause for the relief of distress; rather, its merit lies in expelling Satan from the house in which it is recited. Slapping the cheek is impermissible. We advise you to persist in ruqyah shar'iyyah (Islamic exorcism/healing).