billing and contract issues

i took a personal loan with fnb in march and the contract stipulated that i will pay 1578,96 every month on the 1st for 6 months starting 01-04-2018 then the 1st installment of 1670+- for the 1st 3 months i then contacted fnb i was informed my debit date is on the 115th of every month i then contacted hello peter then fnb called to apologies and rectify the mistake and informed me that everything is fine and my installment will be back to 1579 and on the last month they will adjust the final installment to a lower since i paid more for 1st months. i then asked them to an email stating all that but i was told they cant since they dont the exact figure for the last month. On Monday the 17th September i called fnb to inquire bout the balance due on my last installment and i was advised that it is 1870+-. I then asked about the contract and they told something different to what i signed , i ask the consultant to confirm if the contract in her position have my signature but she refused to confirm , i then asked for a copy she promised to email but it never happened till today. can someone tell me why was the new contract put in place without my concern and why every time i call to inquire why my final installment is soo high i get different answers , and not a direct answer.