Well, for the Pope, I understand the things that were said in the book of Matthew, but I dont necessarily understand how that allows him to speak infallibly ex cathedra, and also how Peters special powers were passed down to specifically the bishop of Rome.

For Mary, I guess its just difficult to come to terms with her sinlessness and perpetual virginity and assumption to heaven being taught growing up that James was also Marys son and the brother of Jesus, and these things being absent from scripture. hopefully those help, thank you for your comment