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You are wrong.
Unless the note provides something unusual, the agreement was that you would continue making payments until paid in full. That would be like buying a car and, if you don't use it, claiming that you don't owe any payments. It doesn't work that way.
But if you have been making payments on a $15,000 note for 17 years, I don't understand how the person who holds the note claims you still owe $15,000. Did you not pay down the principal?