Setting Boundaries With People in the Moment

We all have this notion that you're supposed to set boundaries with someone in the moment, right? But there's also something that's really important that we tend to lose sight of. So a minute later, if you realize that this is actually not ok, you can actually tell them a minute later,. And i know it sounds kind of weird, but, like, why don't we do this? Is because we feel like idiots. Ecause we as human beings, don't like to be contradictory to ourselves. Once i say something, that's the hill that i have to die on, right?