Is believing the thoughts of obsessive doubt, such as "If you do this, something will happen to you," and that they are harmful, considered polytheism (shirk) against Allah?

Even if believing such ideas is not polytheism that expels one from the faith, it is nevertheless a highly reprehensible matter; because it plunges its adherent into a spiral of whisperings and suffering that corrupts his religious and worldly affairs. Therefore, a person must distance himself from these whisperings and ignore them, and let him know that what befell him was not meant to miss him, and what missed him was not meant to befall him, and let him put his trust in Allah, having good expectations of Him.