1. Describe the argument you have already had six times. What is the surface issue? What sentences do you both reliably land on?
2. What are you trying to get them to understand about you? Underneath the complaint is usually a claim about how you feel in this relationship. Write that claim, in one sentence.
3. Which feeling is the anger saving you from? Anger is often the cheaper option. Hurt, fear, embarrassment, and disappointment all cost more to admit out loud.
4. Write the request. One sentence, with no complaint hidden inside it. "I need you to stop being selfish" is a complaint wearing a request's clothes. Ask for one specific thing you actually want.