“No, but thanks for asking” is a sentence worth practicing until it stops feeling rude.

Refusing feels like an act of aggression when you are not used to it, so people agree to things and resent them later, which costs everyone more. A no with warmth in it is not rude, and the discomfort you feel is habit rather than evidence. Most people take it fine. The awkwardness lives almost entirely on your side.

Practice the short version on something low stakes this week, a shift swap or an invitation you do not want. Say it once, kindly, then stop explaining. The silence afterward is the hard part, and sitting through it is the whole skill.