Am I sinful for saying, "I seek forgiveness from Allah for polytheism," knowing that I have not committed major polytheism, but I suffer from whispers of disbelief?

There is no objection to seeking forgiveness for polytheism (shirk). Indeed, the Prophet, peace and blessings be upon him, commanded Abu Bakr, may Allah be pleased with him, to say: "O Allah, I seek refuge in You from associating anything with You knowingly, and I seek Your forgiveness for what I do not know." It is necessary to repel whispers (waswas) and turn away from them, and to engage in the remembrance of Allah, recite the Qur'an, preserve congregational prayers, and increase in seeking forgiveness and supplication.