Daylight *******

Opened an account last year, I did not use all the amount that was credited to me. I paid the monthly amount we agreed upon a month later the amount was adjusted and lowered. Through the months the amount kept getting lowered even though we agreed to a specific debit order amount. It has officially been a year and it seems as though I still have 1 extra year to go even though I did not authorise this reduction. Why was this done without my consent when I agreed to pay the fixed amount? I am very annoyed by this because I should be done paying if it was reduced constantly. Even when I paid extra it would constantly be reduced.