You believe that Jeremiah 31 has already been fulfilled? Well,you believe wrong. In the following passage we can seethat this prophecy has not yet been fulfilled:
"Behold, the days come, saith the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah: Not according to the covenant that I made with their fathers in the day that I took them by the hand to bring them out of the land of Egypt; which my covenant they brake, although I was an husband unto them, saith the LORD: But this shall be the covenant that I will make with the house of Israel; After those days, saith the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people. And they shall teach no more every man his neighbour, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them unto the greatest of them, saith the LORD: for I will forgive their iniquity, and I will remember their sin no more" (Jer.31:31-34).
We can see that the "fathers" of those who will belong to the house of Israel and the house of Judah in the future are those who broke the LORD's covenant. It was the physical descendants of Israel who did that. Therefore, since their fathers are the physical descendants of Israel then all those of both houses will also be the physical descendants of Israel.
And all of them, from the least of them unto the greatest of them, will have their sins forgiven and be saved. Since this has never happened in the past we know for a fact that the fulfillment of this prophecy remains in the future.
Of course these facts are way above your head and you will continue to teach that this prophecy has already been fulfilled.