Is the prayer of one who strives against himself and maintains his various forms of prayer, supplication, contemplation, and helping others accepted, even though he commits the sin of being alone with his fiancé, sending him pictures, and engaging in physical transgressions between them, despite their repeated attempts to stop?

If the marriage contract (aqd) has not been performed, then you have committed sins such as being alone with your fiancé (khalwa) and exchanging photos. The first correct step is to cut off communication with him, because he is a stranger (ajnabi) to you until the contract is concluded. A good intention does not justify disobedience. Hasten to sincere repentance and increase your seeking of forgiveness (istighfar). Your prayer is fundamentally valid and is not nullified except by the realization of its nullifiers. What you feel about hypocrisy is a satanic whisper. The acceptance of prayer is an unseen matter that cannot be definitively ascertained. After the legitimate marriage contract has been performed with its pillars, privacy (khalwa) and communication become permissible.