There’s two tactics that are very useful in preparing for an important and difficult conversation.
First, write your thoughts down. It is unbelievably useful to get what you think out of your mind. That’s how you fill the gaps in your reasoning, how you complete the story you want to tell, how you make sure that your arguments are solid. And in particular, that’s how you avoid the conversation you are about to have from being a stream of thoughts that might be difficult for the other person to interpret.
Second, talk about the issue with somebody. Other people are a great sounding board for your thoughts, feelings and ideas. They add perspective to what you have to say, and they identify the flaws in it much better than you might ever do. They are your first audience, and you should pay great attention to how they react, what words or points they focus on, and the counterarguments they make.