it (validating vs boundaries) may be a matter of degrees. you can also validate without apologizing if you truly have nothing to apologize for. im a big believer in owning ones actions, but there is no apology necessary for being hurt by something someone says.
And after that I took a stand. But I got tired of her not talking to me and saying things like she would cheat if she is not happy. So I just went back to validating. And strangely she stopped insulting me. This was the first time in several months.
what did taking a stand look like/entail?
boundaries are for you, and not to control her behavior. the threat of cheating is a reaction (possibly even an extinction burst) to your boundary. if a boundary is to be a boundary, these threats or her potentially following through are something you may have to face, but thats certainly something that can be tackled separately.