Contract terms changed

I took a loan on our home. More than 2 years ago. The premium calculated was approx R1300.00. I received two letters thereafter confirming increase in premium. To date my premium is R1519.00. Two weeks ago I receive a letter notifying me the contract signed had an calculating error so they have now reviewed my contract and the premium will now be R2400.49. I contacted bank and ombitsman who is now trying to solve the problem as I am on disability pension and cannot afford the high premium. This month the minute money went into my account the bank just deducts the new premium while the matter is still pending to be resolved. If i had known at commencement of contract the premium would be so high I would not have continued with the loan. Now I am really in a predicument. As far as I am concerned it was a mistake from their side and they have now just broken the original contract. And to crown it all I asked for a fixed rate which they said they done, but in actual fact according to premiums from date of contract it does not seem so.