In a situation that is pretty much beyond help, small victories can make you feel really good.
For instance, if your ex-husband's girlfriend is sitting in the car and trying to avoid having any contact with you, it can be quite satisfying to stand next to her on the pavement, watching her studiously reading a receipt to avoid making eye contact with you. If you then keep your ex talking for a while, moving closer and closer until you're right next to and slightly in front of her, where there's no possible way she couldn't realise you were there, it can actually be hilarious to watch her
still reading the receipt, and faking an increasingly happy smile.
There comes a point where it just wouldn't be feasible for her to turn around and pretend she hadn't seen you earlier, so the next trick is to suddenly and without warning open her door and say 'hello' in a really cheerful voice. That's when you get the 'deer caught in headlights' look, her mind goes blank and she forgets how to say 'hello' back. After a second or two, she will grin awkwardly, at which point you beam back and shut the door. Then, when they drive off, you can feel safe in the knowledge that she's feeling rather embarrassed.
Yes, situations like that can feel surprisingly satisfying.