Right, once you have received the initial form you then become liable to pay from that day onwards, failing to pay or delay will only add up to arrears and other problems, okay that said you say you are paying regularly, I hope this is traceable ie. through a bank, DO NOT pay cash direct to your ex, if it is not traceable as maintenance payments, the CSA will not see you as paying anything and you will still be liable.
There is no excaping the CSA now unless your ex stops the claim, so if I were you I would go along with providing the details as you can, continue to pay, until everything is sorted and a maintenance payment has been arranged with the CSA, and take it from there.
From what you say being self employed and having a trading loss, your ex may lose out!! CSA for self employed is assessed on your trading profit, so if you earn nothing you will be assessed for payment based on literally nothing.
There is some details about self employed and payment calculations on the CSA website so go across there and take a look, but don't delay with the CSA paperwork, it will not go away through ignoring, and good luck.