Is there a sin upon one to whom Satan whispers that he is praying out of ostentation, and who then says, "I will not pray" in a moment of anger, and then immediately seeks forgiveness for that?

Your statement "you will not pray" — if it resulted from intense anger that made you lose your mind, then there is no sin upon you. However, if you said it while fully conscious and with a determination to abandon prayer, then you have committed a sin. Nevertheless, your sincere repentance erases what preceded it. You must maintain your prayers and acts of worship, for happiness and success are only achieved through obedience to Allah.