So....STBXH was a very abusive man. No physical but emotionally had me at a point of believing that he and my babies would be better off if I topped myself. Sexual manipulation, financial, cut me off from friends and family. Porn addict. He left (I believe) thinking that it would finally shut me up, that my final shred of resistance to his abuse would shrivel...so that he could march back in as master-of-all-he surveyed.
I have had no support since he left. Family hopeless. Some shred of dignity wouldnt allow me to let him come back without serious changes. So he never got back in 
Since then, one of DC has been diagnosed with SN and I have concentrated on that. Access and maintenance is on an informal, agreed between us basis. Actually, that's not true....he takes what access he wants and that suits him and he give what maintenance he wants.
I have to set boundaries to him for everything (suffice to say the poo-ing ex thread here was no surprise to me).
So, I had a text from him today, to say that he wouldn't see the kids according to the schedule over the next week. No explanation, certainly not looking to see if it's ok. Just as if he was ringing a childminder.
I've had enough of this...I had wanted to do a SN related course next weekend but wont be able to now. (Bet he knew this
). I cant make him be a parent but he is so manipulative that he can twist any assertiveness to find a ''poor him'' angle.
I dont want to rile him. I dont want to go all guns blazing, it's not my style.
So, your suggestions please on how to be assertive, without being aggressive?