A boundary doesn’t need a speech. “No, that doesn’t work for me” is the whole paragraph.

Long explanations turn a decision into a negotiation. Every reason you offer is a surface somebody can argue with, so the more you justify, the more openings you hand over. A short answer has nothing to grip. It can feel rude the first few times only because you are used to paying for your no in advance.

Say the short version once this week and then stop talking. Resist the pull to fill the silence with reasons. If someone pushes for an explanation, repeat the same sentence, which is usually where it ends.