Complaint sub-issue: Trouble with how payments are handled

During my XXXX and XXXX programs at University XXXX in XXXX, Texas, I received several private student loans to fund my tuition and expenses. Upon graduation, I began paying back my student loan debt through the company I borrowed the money from, XXXX. Earlier this year, XXXX began having American Education Services manage/service all existing student loans and XXXX discontinued providing private loans to students. 
While XXXX was servicing my loans, I never had any problems/issues with how my payments were handled/distributed among the different loans that I was paying towards. I was able to pay off several of them by making additional principle only payments in addition to the direct debited amount that was taken from my checking account each month and distributed among my loans. 
Since AES ( American Education Services ) has taken over the payments, I have had to contact them every month, often times several times each month, due to my payments not being posted/allocated as requested. I have sent in multiple email requests, phone requests with customer service agents, and most recently, handwritten requests that were signed, dated, and faxed in to avoid the conflicts with dealing with the customer service agents on the phone. 
I continue to have problems with AES not distributing my payments properly. I am making additional payments outside of my monthly direct debited amount in order to pay down my highest interest rated loans and every time I make an additional payment, AES skips or lowers the amount that is SUPPOSED to be directly debited and either lowers the monthly debited amount due or applies it to the loan with the lowest interest rate. 
I have requested XXXX different times to CANCEL the " paid ahead status '' that AES kept placing on my account, which is what excuse they provided for me when I first made the complaint. The next month, I made an additional payment and my scheduled monthly direct debit amount for the highest interest rate loan was once again skipped. When I contacted AES customer service again to determine why it was happening again after I requested the paid ahead status to be cancelled, I was informed that when a borrower calls to request cancellation of the paid ahead status, it only cancels it FOR THAT MONTH and then it is set to restart the next payment cycle. I was informed that I would need to send in a written request that clearly states I wish to PERMANENTLY CANCEL the paid ahead status on all my loans, which I did. Now, after making several additional payments, my loan with the highest interest rate of XXXX % that shows to have a monthly payment of {$28.00} applied with my direct debit payment, was only paid on in the amount of {$2.00}. 
I am not sure what else to do, because every time I contact someone at AES, I get a different excuse, a different reason, or a flat out lie, on why my accounts are not being paid back in the manner in which I have requested. I have paid back over $ XXXX to my student loans over the last several years and I still owe almost $ XXXX. I do not wish to continue to have to deal with AES mismanaging my money for the next several years to come.