Standard bank loan

I took a house loan with Standard bank of 585000 Rands, which I had to pay over a period of 30 years . I took this loan during March 2011 .I have been paying through a debit order .My debit was 5700 Rands and my employer contributes 1000 rand per month and this will go on till the loan has been payed off . This amount amount increases annually as the salary increases . According to my simple understanding the money paid extra is mine . The bank did not decrease the installment when I was paying more but when I requested the additional money to be paid the installment increased drastically, the bank gave me excuses that didn't make sense . I want the installment to remain the same and I want them to reimburse the money paid extra (the bond money) by my employer. They have been calling me daily accusing of having missing areas . If possible please investigate this matter