Find your own limit before someone else finds it for you by crossing it.

Limits discovered in the moment are discovered too late. If you have not worked out in advance what you will not do or accept, the decision gets made under pressure, usually by whoever is pushing, and then you are arguing from behind about something you never actually decided. Knowing your position ahead of time is what makes it holdable.

Think through the situations that keep recurring and settle now where the line sits. Say it out loud once, to yourself, so it exists in words. A limit you have already located is much easier to hold calmly.