CSA payment just become self employed

Sorry if this question has already been asked before..

In short, my ex and I got back together a couple of months ago, we agreed to send our son to private school, fees were okay as we were living together. Now we split up (the 10th time)., she is demanding I pay his school fee, £400 (which is half) and CSA.

I want to pay what I am meant to, (as this would be around 500£/pm) but I am now self employed, and have no idea how to calculate my income?

Yes, I know I pay 15% of my salary, dividends etc. BUT I need to pay self assessment tax at the end of the year. Does anyone have any advise please on this?

I have always made payments to her in the past, even when not working for periods of over 6 months (I took loans out)..