I had to do this. I had been to court endless times - ex's way of keeping me under his thumb - and legal aid refused to fund any more, so I had to represent myself. I was terrified, literally inwardly quaked. I won! Hands down, he got nothing. First time in all those court cases - and last, as ex never took me to court again.
the court staff are usually very kind and helpful when someone is representing themselves. Do your research, know your case like you're swotting for a doctorate (you probably already do but know it like the back of your hand, every single part of it - and research around it). Keep absolutely calm and factual; if you are questioned keep your answers as short as possible, succinct, to the point - don't drone on or his counsel will use your woolly answers to try to catch you out. Barristers are bright - be bright too, sharpen your brain with all that studying and research. Don't reacte with irritation or frustration to any comments made by ex or ex's counsel - don't reacte. In short, take your head into the courtroom, not your heart.
Last and not least, wear a skirt and look feminine - not OTT but demure/plain. shocking eh (and people may howl with derision but this was my experience, and I had plenty of it) but best to be on the safe side. Find out how the judge likes to be addressed (ask the court staff); be quietly respectful and not ballsy; don't be a know-all. If you can't remember what you were going to say, it's ok to pause - fiddle with your papers or something: better to be quiet than to gabble. Remember to keep your breathing steady - deep breaths, into your stomach.
NOt expert advice, certainly, but bourne out of experience.
Good luck OP. Get some counsel about stopping him endlessly taking you to court as my ex did.