Lack of knowledge on product

I have a student loan with FNB which I regret taking, for the past months I've been paying interest only not knowing that I have to pay more than double what I took. I took R20 000 and now I have to pay R26 000 extra for the period which I did not agree to. I regret being a client of FNB because I was told that for the R20 000 I took I will repay it as R26 000 and I agreed to that and now I have paid R19 000 already and I'm being told I still owe R17 000. This is an unfair treatment. Selling a product and giving wrong information is not right for customers coz now I have to pay my money because someone with lack of knowledge put me in such a mess. My days as FNB customer are numbered, all accounts that I have with them are closed as in this week, I'm done with FNB.