How do I practice standing up for myself without turning it into payback?

You don't have to welcome people crossing your line to get practice. Try a short, clear response: “Please stop,” or “I’m not okay with that.” If it continues, say what you’ll do next, like ending the conversation or leaving. The goal is protecting your boundary, not making them regret testing it.

It may help to decide your response before you’re upset. Pick one sentence and one reasonable next step, then keep both simple. You can be angry and still choose not to retaliate. If the situation feels unsafe, focus on getting distance and support rather than confronting the person.

Also, assertiveness doesn't have to wait for someone to cross a line. You can say what you prefer early, while things are still calm. That gives you practice without needing a conflict to become your training ground.