Is it considered Shirk for a person to say to themselves, after seeking refuge from Satan: "I don't think he will leave me"?

Uncertainty about the supplication being answered is not polytheism (shirk). Rather, one must improve their opinion of Allah and be certain that He is the best guardian and the Most Merciful of those who show mercy. So, if you seek refuge with Allah from Satan, then trust in Allah's sufficiency for you, and that He will repel his cunning and malice from you. Among the most important reasons for the supplication to be answered is certainty of the answer, as the Messenger of Allah, peace and blessings be upon him, said: "Supplicate to Allah while you are certain of being answered."