Product: Mortgage

I recently went through a chapter XXXX bankruptcy which began XX/XX/XXXX and was discharged XX/XX/XXXX. My mortgage was not part of the bankruptcy, and all mortgage payments were paid in full and on time. Initially with XXXX, the mortgage was sold ( or transferred ) to XXXX a year into my bankruptcy. XXXX immediately accused me of owing an additional payment, which was not true. I have all of my reciepts and the release from XXXX which clearly shows I was current before and at the time of the transfer. When XXXX realized I was under chapter XXXX protection, they dropped their allegations against me. I paid XXXX every month - never late, which I can clearly and easily prove with my reciepts of all the payments I sent to XXXX. Shortly after my bankruptcy was discharged, XXXX sold my mortgage to NationStar Mortgage. NationStar immediately ( yes, again ) accused me of missing a payment. I responded to them by sending copies of all of my payments - beginning several months before XXXX sold the loan to XXXX, so there could be absolutely no mistake that my account was current from before XXXX sold the mortgage, to the date of the transfer to XXXX, through the entire time XXXX was the servicer. In all this time, representing over 2 and 1/2 years, my receipts as well as the release from XXXX show clearly and quite simply that I am not and was not late, and I have not missed a payment. So, I am told the issue is that I did not make the first payment to NationStar, and that is somewhat correct, because I made the payment to XXXX. It was only after I made the payment to XXXX that I received notice that XXXX had sold/transferred the mortgage ... I believe this was intentional on their part so they could get that extra payment and not forward it to Nationstar. I do not know if that is the case. This is not something I would have had any control over, at any rate. However, I do know, and I can prove with absolute certainty that I am completely current and I can easily prove this. I believed that once NationStar looked at my receipts that they would see that they were wrong and correct this. I was wrong. I have spent the last 2 months trying to get them to fix this, as well as repair my credit, but they are asserting that I owe them and they have this week added late charges to this fictional payment they claim I owe them. They could easily see they are wrong if they just took a look at my receipts, but they are not interested in my proof. Rather, they are relying on their own paperwork and XXXX 's paperwork, the entirety of which is clearly proven to be false by my records. 
In fact, they are arrogant about this, with the latest Nstar employee stating to me that they consider the matter closed. 
I obviously do not have the power to compel them to fix this, and I get the feeling that this is on purpose by them so they can begin to attempt to mine me with this fictitious charge, late fees, and whatever else they will begin to attempt to collect as time goes by. I believe they are corrupt and this is just their mode of operation. 
But, I am only one person, with limited resources, and apparently with no way to compel them to do the right thing. So, my credit is showing late payments, I can not even consider refinancing my mortgage because they have trapped me in their company, and any actions I take that involve someone checking my credit will be damaged by the fraudulent information on my credit report that NationStar is actively maintaining. 
I believe NationStar is a corrupt company, and they are trying to hurt me just for profit.