The first time you say no to your boss and the world doesn’t end, something in you relaxes.

The fear of a first no is usually much larger than the event. You expect consequences, and what generally happens is a short pause and someone asking the next person, because a shift being covered is a logistics problem rather than a judgment on you. Afterwards the fear is permanently smaller, since you have evidence now. Most of the dread was imagination.

If you get asked for something you genuinely cannot do, say so plainly once: I am not able to on Saturday. No reasons stacked up, no pile of apologies. Then notice what actually happens, because that is the thing that changes you.