ACS Student Loan/MRS collections

Hi there,
I will try to sum this up as quickly as possible. I have a very large "student loan" through a company called ACS. I took it out at about $17,000 and it is now at 21+,000. After I graduated college ACS wanted me to make very large payments that I could not afford. I tried making partial payments only to be charged $25 fee for not paying in full. In short, my monthly payments go up, never down. I have tried to work with ACS for over a yr. now to try to have my payments lowered or create some sort of payment plan. I submitted my request in writing and was denied.I tried asking who my lender was so I could contact them and they gave me the wrong lender! Every time I spoke with them, ACS informed me that my ONLY option is to pay what they are asking no matter what. But, what if I can't!?! So, in Dec. I moved home and did not have a job, I couldn't pay them and tried again to work something out, no such luck. With all the loops they have thrown me through I gave up and stopped paying. Not smart I know but I didn't know what else to do, either way my bill was just going up and up. Now MRS has contacted me by letter. So I called the phone # they listed and spoke to a man named Andi who informed me I was being sued! He then informed me that I had to pay him $391 right then plus $6 service fee immediately! So I did. I was so scared, but he said if I didn't I would be sued and taken to court. He said the $391 was to stabilize my account and to stop any further action. Then he informed me that I have to pay $6,931 dollars by Monday!!! He kept asking if I could borrow it from my family and just kept saying I had to pay it by Monday. I don't know what to do! I called MRS again today to figure out where these amounts came from and what happens if I can't pay the $6,931 by Monday? I spoke to Andi's again as well as his manager Christina who treated me like a criminal! I called to authorized my dad on my account so maybe he could decide if I am getting screwed or tell me what to do and Andi and Christina both got very upset about that. Asking me why he would talk to them and Andi said flat out that he would not discuss the subject of payment with him. Christina told me only minors have their parents call and that I was acting like a child. I tried to explain to her that I have NEVER been in a situation like this and am just trying to figure out what is going on and what I should do. She then told me that I knew what was going on and how this happened, and that I need to pay the 6,000+ or I am going to court and will have to pay the 21,000 immediately + lawyer/court fees! I don't know what to do! I am 25 yrs old, have never owned a credit card or had any kind of bad debt like this!! I pay all my bill and I have 2 other student loans that I pay because they work with me like a student loan company should. MRS even said I had impeccable credit before this. I am terrified and am not sure what to do. I am sorry if I am rambling, this all just happened yesterday and so fast. I have no idea what my rights are or what the right thing to do is. If you have any suggestions I would greatly appreciate it!
Thank you so much for your time.