Bad service and handling of accounts

I am very disappointed with the service I received After paying faithfully the last 5 years I struggled to get my clearance papers. I had to constantly send emails en tried to call to have someone assist only to be told that I am not done paying and then continued to deduct money from my account with no agreement from my part to do so. I tried to get answers as to why I still need to pay after according to their statement I had paid off everything. I then learned that Fnb continued to charge me interest on my debt and it was never calculated into my payments I made the last the 5 years. The response I got was to say after an additional 2years of paying this off will I only see a decrease in my capital that I owed. My debt has doubled I the last 5years instead of decreasing.