Check 'N Go Installment Loan

I have a Check N Go installment loan that I have been paying on forever. The payments are very high and I got behind. I was two payments behind. I tried to talk to the store manager but she basically screamed at me to get it paid and hung up. I made a partial payment toward the two payments but she said it was not enough. I am new to this site but have read a little on here about Check N Go installment loans but am still very confused. When you make a payment on your installment loan, each time you make out a new check for the balance due and it is dated for a future date. The manager said they would run my check and when it came back NSF I would be turned over to collections. However, I have made a partial payment since then so I am not sure what is going to happen. She said the partial payment was not enough but she did accept it. I have tried to talk to her several times and she just yells at me. I cannot afford to make the next payment. Right now I am just one payment behind since I made a partial payment but will be behind two payments when I cannot make the next one. Can anyone give me any advice on what to do or what might happen? I tried calling the national Check N Go number but they referred me back to the local store. It is impossible to talk to the local people. I would appreciate any help. Thank you.