The following is a verse from Isaiah (50:1) which helps us in our understanding this verse:
כֹּה אָמַר יְהוָה, אֵי זֶה סֵפֶר כְּרִיתוּת אִמְּכֶם אֲשֶׁר שִׁלַּחְתִּיהָ, אוֹ מִי מִנּוֹשַׁי, אֲשֶׁר-מָכַרְתִּי אֶתְכֶם לוֹ; הֵן בַּעֲוֹנֹתֵיכֶם נִמְכַּרְתֶּם, וּבְפִשְׁעֵיכֶם שֻׁלְּחָה אִמְּכֶם.
Thus says God: Where is the the bill of your mother's divorce with which I sent her? Or whom of my creditors have I sold you to? Behold, for your inequities you were sold, and for your transgressions your mother was sent away.
What does this mean? First Isaiah makes it sound like there is no divorce ("where is the bill of divorce?"), then makes it sound like there is ("for your transgressions your mother was sent away"). Is there or is there not a bill of divorce? The way many of the Jewish commentators understand this is that there is no permanent bill of divorce, but rather one that is dependent on certain conditions. "For your transgressions your mother was sent away", implying that when the transgressions cease, the divorce will be revoked.
Now, what about Jeremiah? Why doesn't he specify something like this. Jeremiah implies that there is a permanent bill of divorce that was given. Many say Isaiah was referring to the Kingdom of Judah (as he prophesied primarily about Judah; see Isaiah 1:1), while Jeremiah was referring to the already-banished Kingdom of Israel (as is clear in the context of the verses). Israel was driven out of their land, perhaps never to come back. Judah was to be exiled, but destined to return less than a century later. (Although many maintain that Israel too will one day return in the final redemption, it is not uncommon for prophets to refer to indefinitely long periods of time as "forever".) [Alternatively, God has permanently divorced Israel as a kingdom, in the sense that even when they do eventually return, they will be joined with the Kingdom of Judah, and under Judaic (Davidic) leadership.] (See Radak to Isaiah 50:1.)