What is the ruling on obsessive whispering (waswasah) about insulting Allah and His Messenger (peace be upon him) and uttering statements of disbelief (kufr), and is it considered part of Shirk (associating partners with Allah)?

The fundamental principle is that cursing Allah or His Messenger is an act of disbelief that expels one from the fold of Islam, by the consensus of scholars. It is most likely that what you are experiencing is not actual cursing but rather satanic whisperings (waswasah). Satan whispers to you that you are doing this, and there is no accountability for waswasah, based on the saying of the Prophet, peace and blessings be upon him: "Indeed, Allah has pardoned for my Ummah whatever they whisper to themselves, as long as they do not act upon it or speak of it." Furthermore, your dislike and hatred for these whisperings are a sign of clear faith. If you are certain that you have uttered the curse, it is possible that the whisperings have led you to a state where you lost intention, and in that case, you are also not held accountable. It is recommended to recite the Quran frequently, especially the Mu'awwidhatayn (Surahs Al-Falaq and An-Nas), Ayatul-Kursi, Al-Fatihah, and Surahs Al-Baqarah and Al-Imran. Also, maintain the morning and evening remembrances and the remembrances before sleep to ward off Satan.