Practice the hard conversation out loud once. Your first draft of words is always worse than you think.

Sentences assembled in your head skip the hard part, because you never actually have to finish them. Out loud, you find that the opening is accusing, the middle is muddled and the point arrives far too late. Hearing it once lets you fix all of that while nothing is at stake. The real conversation then starts on your second attempt instead of your first.

Say it to an empty room, into your phone, or to somebody not involved. Keep the opening to one sentence and name what you want. If it comes out angrier than you meant, better to learn that in the kitchen.