Every boundary begins as a decision before it becomes a standard.

A boundary starts as one uncomfortable moment. You say the thing once, badly perhaps, and only after several repetitions does it stop being an act of nerve and become simply how you are treated. People learn your limits from what you enforce, not from what you intend, and the first enforcement is always the hardest.

If there is a limit you keep meaning to set, treat the first instance as the whole job. Say it plainly, without a case built to justify it, and expect it to feel awkward. Then say it again the next time. The awkwardness fades roughly at the point where the boundary becomes a standard.