People who resist your boundaries are often the ones who benefit from having none. Use this formula: "I'm happy to A, and I'm not willing to B." Say it calm and clear, no apology needed. Silence gets mistaken for yes.
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The people that tend to complain about reasonable boundaries being in place are the very same people who benefit from them not being in place, or being in place but vague and open to interpretation. And have you noticed the interpretation they make is always presented in their favour? Here's a boundary phrase no one can really argue with. It's simple. This is the pattern: I'm happy to A, and I'm not willing to B. So, I'm happy to discuss this with you, and I'm not willing to do it when your voice is raised. Or, I'm happy to hear your view, and I'm not willing to be pushed into agreement by you. It's calm, it's clear, no apology. Boundaries don't push people away, they show people how to engage with you. Importantly, it also shows people how you will engage or not with them, which is a boundary. We teach people how to treat us. Sometimes a little reeducation is required. Be proactive, do it kindly, do it assertively, but don't risk your silence being taken as agreement or acceptance of a boundary that really isn't a boundary.