An angry customer

I bought my car last year and had insurance with auto and general. A month later I cancelled that insurance and opened an insurance with Budget insurance. Wesbank opened a comprehensive insurance on my behalf not even consulting with me. Since last year October I've been asking them to cancel their insurance and during that time I stopped paying for that insurance but continued to pay for the car. In June thia year they finally cancelled their insurance now my problem is, from last year October to June 2018 the insurance money made arrears of R8000+. They agreed to refund me for the months they have been taking the insurance money which is R6000+. They now say the R6000+ they'll use it to pay the arrears and now I owe them R2000+. From June to this month that R2000+ has accumulated interest and it is at R3000+. This morning they called me and said I have missed a month installment and if I don't pay the arrears they will increase with interest and they will have to repossess. Not once have I missed a payment that is on the contract. Please help me.