Product: Student loan

Salutations, Over the past three years I have experienced considerable frustration with Navient student loans.
I took out student loans, through   XXXX   XXXX  , in  XX/XX/XXXX  and  XX/XX/XXXX , with payments beginning in  XX/XX/XXXX . I set auto pay for this account, in order to take advantage of an offer for an immediate loan reduction of .125 % ; bringing my initial APR to 3.375 % - Due to three-years of uninterrupted on-time loan payments my interest rate was subsequently reduced to 2.125 %.
Just prior to the   XXXX   XXXX   transfer to Navient I went online to see what my current loan balance was ; which came to a little over one-thousand dollars. Immediately following the change to Navient I decided to review my current balance ; which had now ballooned to nearly four-thousand dollars. I know that I had never missed a payment, due to the auto deduction I set up back when I first took out the loans, so I rushed to the phone to find out what the   XXXX    happened.
When I finally reached Navient their agent explained that my current balance was the product of a consolidation I made in  XX/XX/XXXX , of {$12000.00}. I tried to explain to the agent that I had never requested a consolidation of my student loan, I had never missed a payment, and my original loan amount was for {$12000.00}. The agent insisted that my information was incorrect and that my current balance was now just under four-thousand dollars. The agent agreed to send me a copy of my loan agreement ; which arrived three weeks later, and was based on the  XX/XX/XXXX  consolidation agreement I never made.
I decided to wait until the term of my loan had ended before reaching out to Navient again ; hoping their misunderstanding was a glitch in their newly established system. When I finally reached out to Navient, at the end of my original loan agreement, I was informed that not only where the standing by their original assessment, but my percentage rate was 3.375 %, compounded daily ; I dropped the phone, called Navient back, and a new agent calmed me down by explaining that the previous agent was mistaken about the interest rate, but was correct about the terms of my loan ; she volunteered to send me out another copy of the loan agreement, which was a carbon copy of the one sent out the year before.
Due to lifes twists and turns the time had finally come for me to address this issue head on. I reviewed my records, performed a simple equation on  XXXX , and came up with a current loan amount of - {$2100.00} ; which I assumed would get Navients attention considering they where now guilty of grand-theft.
I emailed Navient the following : Good day, Per my request, Navient provided a copy of my current loan agreement ; said agreement correctly notes original loan amount, of {$12000.00}, a payment of {$90.00}, and an original APR of 3.375 %, with a total principal and interest of {$16000.00} ; but for some reason said document notes Date Interest Begins to Accrue of XX/XX/XXXX - account  XXXX .
I began making payments to this loan in  XX/XX/XXXX  and have now paid a total of {$16000.00} ; which is substantially more than the {$16000.00} noted in the " Principal and Interest You Will Have Paid '' section, of the   XXXX   Disclosure Agreement. My APR was reduced to 2.125 %, in  XX/XX/XXXX , due to my three years of uninterrupted payments ; which means that I have now been overcharged {$1500.00} - based on my calculations.
I believe the disconnect happened after Navient spun off from   XXXX   ; somehow accounting my automatic interest rate reduction as a consolidation of an existing loan - my loans still show as two separate loans on the Navient Website.
Now, if you could be so kind as to please show me when my original loan of {$12000.00} transformed into a loan amount of {$15000.00} I could stop thinking about how a multi-billion dollar corporation was holding my credit rating hostage - when did my fifteen year loan transform into an eighteen year loan?
Im trying to be calm here ; but I see nothing that tells me why my payments from  XX/XX/XXXX thru  XX/XX/XXXX  count for nothing.
Thank you for your time and consideration.
Note : this my third attempt to reach out to Navient regarding this issue  XX/XX/XXXX  Navient responded via the attached pdf.doc ; maintaining their previously stated position, and asserting their right to an additional {$1600.00} bringing their potential theft to about {$5000.00}.
I contacted Navient when I got home from work that evening and after having to endured the same explanations I had received the previous attempts I asked if the agent would be so kind as to tell me what my original loan amount was back in  XX/XX/XXXX . The agent was kind enough to review my records, of the past sixteen years, and informed me that my original loan amount was in deed {$12000.00}. I then asked the agent how my original loan could balloon to {$12000.00}, after three years of payments, to which the agent began to explain that there was a disbursement made in XX/XX/XXXX to your schoolwaitoh my The agent assured me that a form would be sent out in the few weeks explaining why Navient did what they did.
At this point I want more than just the money Navient has stolen from me, more than interest and penalties they would have charged me, and more than a simple slap on the wrist from one executive to another I still do not know if Navient plans on making good on their obvious and deliberate mishandling of our contractual agreement.
Thank you for your time and consideration,