How can we be sure that the rulings of the authentic (sahih) and good (hasan) hadiths available today have not been abrogated after their narration, and what guarantees us that there are no abrogated hadiths of which we are unaware?

Abrogation (Naskh) is the annulment by the Lawgiver of a previous ruling with a later one. Abrogation is recognized through several means, including: an explicit text from the Lawgiver or a Companion, the consensus of the Ummah on abrogation, or the contradiction between two hadiths with knowledge of which is earlier and which is later. If two hadiths contradict each other, and it is possible to reconcile them, then both must be acted upon. Abrogation is resorted to only when reconciliation is impossible. If the two hadiths are contradictory and cannot be reconciled, the abrogating one is given precedence if known; otherwise, the stronger of the two is acted upon using the various methods of preference.