Shabbos Nachamu

Can you please explain the meaning of Shabbat Nachamu? I heard in a class that there are three weeks of Haftara portions leading up to the day of Tisha B’av and then seven weeks of portions of comforting, starting with Nachamu. Why would there be so many more weeks of comforting, and how could we be comforted immediately after the destruction of our people? Could you please provide some explanation or meaning to this period; it would be appreciated!