How do we reconcile the saying of the Prophet (peace be upon him), "If I live until the next year, I will fast the ninth," and his death before fasting the ninth, with his residence in Medina for more than ten years?

There is no contradiction between the hadiths concerning the fasting of Ashura. When the Prophet, peace and blessings be upon him, arrived in Medina, he agreed with the Jews in fasting Ashura and said: "We are more worthy of Moses than them." Then, when Islam became manifest and strong, he desired to differ from the People of the Book, so he said at the end of his life: "If I live until the next year, I will fast the ninth (of Muharram)." This indicates his intention to add a day before it, in divergence from the Jews. The Prophet, peace and blessings be upon him, initially liked to agree with the People of the Book, but then preferred to differ from them after Islam gained strength.