The more you overexplain a boundary, the more it sounds like a suggestion.

A long justification signals that you are waiting to be released from the position, because reasons invite counterarguments. Every extra sentence adds a place to push, so the other person hears a negotiation opening rather than a decision closing. Length reads as uncertainty even when you are completely certain.

Say it in one sentence this week and then stop talking, even if the silence is uncomfortable. No, I am not able to do that. Let the pause exist instead of filling it with reasons, and notice how much less pushback comes back at you.