A request is not a boundary
A boundary is a decision about your own behaviour. It is not an instruction to somebody else, which is where most of the confusion starts.
"Please stop asking me to work weekends" is a request, and it depends entirely on the other person agreeing. "I do not pick up work email at weekends" is a boundary, because it describes something you control. The second one holds whether or not anyone approves of it, and that is precisely why it works.
Why they can feel so uncomfortable
If warmth tended to arrive when you fitted in with what was expected, and cooled when you did not, then being accommodating stops being a choice and becomes the way you are habitually in relationship. If that is familiar, a boundary will not feel neutral. It feels like putting the relationship at risk.
The guilt is usually loudest in the first hour after setting a new boundary, and it is commonly read as proof of having been unkind. It is worth being slower than that. Sometimes guilt marks something unfamiliar rather than something wrong.
What they sound like
Boundaries land better when they are short, specific, and about you. Long explanations tend to invite negotiation, and over-apologising signals that you expect to be talked out of it.
- "I am not able to take that on this month."
- "I would rather not talk about that."
- "I can stay for an hour, then I need to head off."
- "I am going to step out if this gets heated."
When someone reacts badly
Some pushback is normal, particularly if you have historically been very available. A relationship adjusting to a change is not the same as a relationship being damaged by it.
What is worth paying attention to is the pattern. If a reasonable, quietly stated boundary is consistently met with anger, punishment or withdrawal, that is information about the relationship rather than about whether the boundary was fair.
Keeping one is harder than setting one
Most people can manage the sentence. What can be difficult is the time that follows: the urge to soften it, to explain again, or to make up for it by being especially accommodating about something else.
That urge is the old pattern trying to restore the previous arrangement. Sitting with it, without undoing the boundary, is where change can happen.