I'll clarify the question. Here's the scenario: Person 1 and person 2 are in a relationship. Person 2 breaks up with person 1. Person 1 is hurt and doesn't want the breakup, but they act as if they did (for example, "that's your loss," "didn't want to be in that relationship anyway," etc.).
The sentence would either go like: Person 1 acted [adverb] after the breakup. OR Person 1 [verb] after the breakup.
There's a similar question with answers like "feigning delight" but I'm looking for something with almost a spiteful connotation. I'm also looking for something in more formal language, i.e. "sour grapes" isn't cutting it for me right now.