Credit Card ridiculous interest

Standard bank credit cards it's like a nightmare. How come you pay the installment every month and you don't even use the credit at all but the installment flactuate by going up and down...and it remains on the same thousands only flactuate on the hundreds and tens, is it this ridiculous interest or am I getting ****** by the standard bank. In Jan 2020 I was owing R48882.52 my installment is from R1500 to R1300 every month. In Feb 2021 the amount is R40264.42 then I paid R1300 installment. March 2021 the amount is R40834. 89 and I even destroyed the card but it doesn't decrease. So basically from January 2020 to March 2021 I have paid only R8000 and R11000 its the interest.. Can someone please explain this. This is confusing and frustrating Because even if you enquire you don't get full explanation Standard bank tortured me for years with this credit card..and I bank with them I even decided to stop banking with them because I am not happy at all, it's years they have been robbing me like this I would not even advise anyone to attemp banking with this Bank