Setting a boundary and feeling guilty almost at the same time is an extremely common experience, especially for women who grew up learning that their worth depended on how much they could give, endure, or please.
Guilt shows up because the boundary breaks a familiar pattern. Not because the boundary is wrong. Confusing those two things—discomfort from a pattern change versus actually doing something wrong—is one of the most common traps when it comes to holding a boundary.
A healthy boundary doesn't need a long speech or an exhaustive justification. "I can't" is a complete sentence. "Not today" is a complete sentence. The more explanations you give, the more room you leave for the boundary to be negotiated, and a boundary that's constantly negotiated stops being a boundary.
Setting boundaries with the people closest to you is usually the hardest part, precisely because those are the relationships where you've spent the most time giving in. Expect initial discomfort—it isn't a sign you got it wrong. It's a sign the system is readjusting to a new version of you, and that always takes time.
The question that helps hold a boundary in difficult moments isn't "will the other person like this?" It's "does this respect what I need right now?" That second question is, at its core, another way of choosing yourself.