Overcharging & changing Interest Rate willy nilly

HI I want to bring to you attention, I paid R40000 on the 11 Oct in the hope to bring my monthly payments, down yet you call centre Ayesha says that the fixed amount or loan term is still valid & I must pay R2277 for the remaining period., Which is really unfair , also she says that if the loan amount is below R10000 I pay an interest rate of 19 % instead of 16 % .