Boundaries
The boundary itself is usually the easy part. The guilt afterward is the hard part.
Most people know, at least conceptually, how to state a boundary. What tends to derail it is the discomfort that follows, guilt, fear of upsetting someone, or a family system that responds to the boundary as an attack.
If a family system rewarded accommodation and treated your needs as secondary, a boundary can feel like breaking a rule you didn't know you'd agreed to. The guilt isn't a sign you're doing something wrong, it's a sign the old rule is being challenged.
Boundaries tend to hold better when they're stated plainly, without an extended justification, and when you're prepared for the other person to be unhappy about it without that meaning you were wrong to set it. Practicing this in therapy first, before the actual conversation, tends to make the real one easier.