I think you can cultivate whatever image you want to cultivate about yourself with social media. That doesn't mean it's reality. She likely has extra incentive at the moment to prove something to you, your families, and your kids. Perhaps most importantly, she probably has a lot of incentive to prove to herself that breaking up her family was the right choice. It's self-justification in order to avoid cognitive dissonance.
Cognitive dissonance is that unpleasant feeling that occurs when our actions contrast with our ideas, beliefs, morals, or feelings. It is bothersome in any circumstances, but especially when our concept of self is threatened. When this happens, we feel the need to justify our decision in order to save our sense of self. We've all done it to some degree when making an expensive purchase for example - afterwards to avoid the guilt/negative emotions of spending that much money, we tell ourselves that it was totally worth it and that we are happy to have done so, because the purchase was necessary has all sorts of collateral benefits, etc.
In your ex's case, she made a major decision that has a lot of negative fallout for herself and the people around her. She is a known cheater - and that alone obviously holds some baggage (and is likely contrary to her self-concept of a good person). As such, I'm sure she's desperate to convince herself and others that this was the right choice for her and that you were the one holding her back and that her current life is all roses and rainbows. Maybe she even believes it right now, who knows. I know when my dad left my mom for the AP he certainly had some element of this that looking back felt a bit performative.
New relationship energy is a thing. Everyone is on their best behaviour.
No one is complacent. Eventually though that will come to an end, and the same issues and character flaws (or new ones!) will still be prsent. This is what happened to my dad, the same issues cropped up with his AP that were present in his marriage to my mother, and he recently acknowledged that he was the common denominator in it all and sees now that his unhappiness back then was not because of who he was with. He has a lot of regrets looking back and has since mentioned that he wishes he had done things differently. I'm not sure if your wife is capable of that sort of insight - or if she'd ever admit it if she was - but I think it's probably true.
I know I'm in no shape to be with someone so I feel very alone having no one to talk to or no one to share the joys of parenting with while she has a person to do that with.
This feels very honest and valid and wise. My heart goes out to you - truly. My guess is that some part of her may be feeling the same way you are right now, and that is why she is being so active posting in your family groups because she wants to share with them. I imagine having split custody gives her a break that allows her to be more present/appreciative of the time she does spend with the kids - this is something I've heard from many divorced parents (men and women) that the time away gives them a chance to miss their kids that they didn't have when they were married. I don't think that necessarily means that you never should have been together, but I am not in a position to opine on that in any event. I do know you can't change the past. And also, thos kids of yours that you love, would not exist if you weren't.
Your feelings though, those are valid. Strength to you.
