Unclear interest

I have a loan of R150 000 with Capitec, a monthly instalment of R4888 with 24.5% interest. So last month I made an upfront payment of R14000 and R5000 this month as I want to #WIN this battle of not spending 5years with a loan in my name. I am being charged R2900 as interest and I tried to reach their call centre , so that I can understand why are they charging me this amount and I still don't #WIN as the last week an agent told me that they charge the interest from instalment amount meaning its 24.5 of R4888 (I did calculations and what she told me did not correspond). Today I made a call again and the agent I found told me that she doesn't understand why my interest is so high and referred me to the branch as she might end up telling lies, I then went to twitter for another clarity and I got the response that interest is charged by looking at my credit risk profile. I am actually fighting a loosing battle her and will not #WIN as Capitec is taking me from pillar to post.

Now I want to know, is there such a thing as interest is charged according to a credit risk? My score keeps on improving and I always make upfront payments to my debts. Will I ever #WIN this battle with Capitec or I will keep paying instalment and not #WIN