Consumer suffers for internal errors

Pluase a rental agency handed my account over to DMC for collection with an outstanding amount of R6200 in July 2017 DMC called and informed me that my account was handed over for the collection of the R6200 i acknowledged the money owed and therefore i had made an arrangement to pay R500 monthly for a period of 13 months. After 9 consecutive months of paying i reversed a debit order DMC contacted me again informing me i have 4 more payments plus the payment i reversed bringing my total arrears to 2500 this was in April of 2018. I once again gave my consent to debit an amount of R500 monthly for a period of 5 months. 5 months was to be ending in August of 2018, before the final deduction of R500 was to go off in August i called DMC to confirm this will be my last installment. I was then told i still have an outstanding amount of R2000 and whoever told me that my payment of R500 would take 13 months to pay off my debt was wrong because interest is added at the end of every month i then i informed the consultant i had an agreement that i will pay R500 a month for a period of 13 months and i have done that.DMC then said they will deal internally with the consultant I should just continue paying i asked when will i finish paying if you adding interst monthly DMC said they dont know i must just pay if i dont my account will never be settled. I made an arrangement with them i paid in accordance with this arrangement now all i want to know is how long will it go on for or how long will i still have to pay an amount of R6200 after paying R500 for a period of 13 months. Its not me who said it would take 13 months why should i suffer for internal mistakes