Does anyone else have trouble dealing with a narcissistic/entitled ex?

I find that the ex often tries to make demands with regards to what suits her every few weeks. We have a voluntary mediated agreement with regards to child care and seeing the kids etc. But quite often I have to pick up the slack and go the extra mile because she has "insert this weeks issue here". Even the kids have started to notice this pattern and make jokes about how Mummy always has something wrong with her when it suits her, and so has to be accommodated.

She cheated in the relationship and even then I had wanted to try to move on but she wouldn't give it a try. So, its not as if this is a situation that has been forced on her, so she has little reason to feel the injured party and be so entitled.

Each time I tend to not fight back or hold a boundary because if she doesn't get her way, or if I even show annoyance at her suiting herself all over everyone then she gets more belligerent and acts hard done by, which just makes things worse.

Does anyone have any tips on how to deal with this attitude in a way that keep the peace?