crazy interest on loan.

So Im paying roughly R1400 a month of from a loan i made last year and my outstanding balance is still the same. FNB hits me a month with roughly 800 interest on top of my debit order of R1400 and the money gets deducted from a debit order. They also ****ed up the deduction dates the first 3 months after I specifically said to them not to deduct it on the end of the month. I feel I am being ****** and Im sure the interest Im paying is ilegal because I am not at fault for the interest and secondly the interest amount is too high.