Stating a limit is the easy part. Staying calm when they lean on it is the whole job.

Saying the sentence takes about four seconds. What comes afterward is the real test: the sighing, the guilt trip, the same ask in a different voice, the joke about it made in front of other people. That pressure is not proof you got it wrong. It is what it sounds like when something that used to work on you stops working.

Decide in advance what you will say when they push, and keep it to one short line you can repeat. No new arguments, because a new argument invites a new debate. Expect a few uncomfortable rounds, and trust that calm repetition ends it sooner than volume does.