Ruined my credit score & disgusting service

If i could give a 0 star i would, i opened an account end of last year needing only to create a credit score and while opening the account the sales lady instructed that my installments a month would be R170 which i was perfectly happy with. After using the account a couple of times & paying my R170 installment i started getting sms's & calls that im over due. I have explained a hundred times to many different people now that the lady that opened my account confirmed my installment which is what ive paid, to which ive been told thats for a 12 month account not 6 so they need to change it to 12 months.... That has never happened!!! Ive emailed, ive been to the store many times the manager has sent forms, ive explained on the phone many times, nothing gets done. Now my credit score is damaged because no one does their job at sheet street, only when its time to phone me about my overdue account does someone contact me!!!!! The service is disgusting!