@Turnedovernewleaf
My ex is a nightmare at parents’ evenings. At the last one, the teacher was explaining that DS2 is doing really well in reading and writing (and she knows that I we read a lot with him) and is doing fine at maths. So ex fixated on the ‘problem’ (because he considers working at national expectations not good enough) and kept asking how to make sure he was ‘working at depth’ for maths too. He just cannot celebrate the positives and starts picking on what he perceives to be negatives.
The really frustrating thing is that ex is shit at maths. He was in the bottom set for maths at school and only just managed to fluke a pass in his GCSE. His parents would have been delighted if his teachers told them he was performing in line with expectations! He also has crap grammar - and what he does know is because I taught him - and never reads outside of work, ever. You’d think all of this would make him appreciate how well DS2 is doing, but nothing is ever good enough. 
He was much the same at the swimming gala. He was clearly disappointed in DS2’s performance throughout - he hesitated a bit at the start, he didn’t win his heats (which included kids 3 years older than him as they mix them all up and then do te results by time), etc. Then the results came in and you’d think he hadn’t been so bloody negative the whole way through.
Luckily DS2 wasn’t there to hear any of it in either case.
@Happycow Thanks. I’ve really found that posting on this thread has helped me to think more clearly and to be more confident. Sometimes, if I’m tempted to be a bit of a doormat with ex, I think of what kind of responses I’d get on here about it and resolve not to let him get away with whatever it is. ‘Would I want to admit to this on the thread?’ is a great test in any situation with ex.
And it is cathartic to share the shit with everyone, and laugh about our terrible exes! (Especially when the alternative might be to despair alone).