[Plans A-Z]

So I have this letter , and it's not the first. We've been in repayment before, fallen behind, gotten a repayment plan that is so hard to keep up with because it adds almost $600 a month to our regular payment, which is already high for us. But we work through it and catch up again. Each summer paying for childcare when the kids are out of school puts us behind and I always think I'll catch up and boom this letter comes.
So I made the call right away, didn't want to do repayment or modification, just wanted more time to catch up. They extended my date from Oct 2 to the 7th.( to pay the total amount due) As the 7th was getting closer and groceries, gas and car repairs took the front seat I saw I was unable to come up with the full amount due on the 7th so I paid what I could ( about half) last week. The payment hasn't posted to my account but it does say it's on it's way to the "payment processing center" scary...to me that means it may be refused.
The man I spoke with after I got the letter was very nice and said now if you can't make the payment by the 7th you need to call us...

So when I call, I'm really nervous about hearing that I can't have any more time etc and they won't work with me. I feel by the end of the month I can send the rest due but I'll still be a month behind.
I want to call and ask but I'm not sure what they'll say. I'm losing sleep!! I don't want to sell, or set up a 6 month repayment, but I can't come up with the amount due by the 7th.

What would any of you do? Anyone special I should talk to? What are my chances they'll give me more time? And does the payment I sent count for anything or will it be returned? I know the guy said the letter gets sent when you are 3 months behind. So making half of the payment , to me, got me less than 3 months behind. But if they don't count it, I'm back to owing the whole amount.

Any help would be so appreciated. These people scare me based on the different stories I've heard here and other places.