Speak to your IP and advise them of the issue. You may be able to take a payment break for a month or two or maybe the repayments can be modified slightly.
If it doesn't look like you will be able to continue with the IVA then it will fail after you miss a certain number of payments, 3 I think. Your IP can then issue you with a certificate showing that the IVA failed and you can apply for BR.
It is 3 paymnents you have to miss,I'v just missed my 3rd and IVA co have sent me I letter to take to court saying they have no objections to me filling for BR. I'm doing that on 18May, saved my iva payments to pay for it.Sorry I didn't do it 2 years ago instead of strugglingin iva for that time.