Please help

I have been an account holder with african bank since 2012 things were ok I until last year where i could not afford full payments an agent was send to me to make arrangements for the payments of the account and we agreed on half the instalment every month signed new contract little did i know was that I will be paying the account forever when I call today so that I can have the interest stoped i was told that they cannot and with amount i agreed on with african bank does not even cover the interest being charged i want to know as to how am I going to finish this debt if the creditor is not willing to help me finish it