A boundary is information about what you can and cannot do. It is not a rule you impose on somebody else, and it is not a way of getting them to change.
Rigid, healthy, porous
A rigid boundary keeps everything out: safe, and lonely. A porous one lets everything in: connected, and depleting. A healthy one is a gate you open and close on purpose, differently with different people.
Most of us are not uniformly one thing. We tend to be rigid in some relationships and porous in others, and the pattern is worth knowing.
Boundary or control
"I'll leave if the shouting starts" is a boundary — it describes what you will do. "You must stop shouting" is a request; reasonable, but not yours to enforce.
The difference matters because only one of them is within your power to keep.
Guilt after saying no is usually proof that it is unfamiliar, not proof that it was wrong.