Before you say it
How to bring up the thing you have been avoiding.
You already know what you want to say. What you do not know is how to say it without the next two hours going the way they went last time. That is a preparation problem, and it is solvable on your own.
Five things worth doing first
Being understood, getting a decision, setting a boundary, an apology, or simply saying the thing and living. These need different conversations, and most go wrong because the person opening them had not picked one.
"You were forty minutes late" is a fact. "You do not care" is an interpretation of it. Both are worth saying — but a conversation that presents the second as the first is one the other person can only argue with.
Almost all of the damage is done in the opening. If you have said them once already, somewhere private, you are not improvising the worst version at the worst moment.
Know before you start which parts you would genuinely move on and which you would not. Discovering that mid-argument is how people concede things they meant to keep.
A pause, a boundary, another attempt on Sunday, or accepting that you disagree. Having an ending you chose is the difference between a hard conversation and a bad one.
None of that needs software. It needs somewhere to think, and someone to think at who is not the person you are worried about.
Where OnBetterTerms comes in
A private guide that knows your history — what you have tried, what set it off last time, how you like to be talked to — and that the other person can never see, or be told exists.
Write the thing as it comes out, say what you want from saying it, and get back a version you could actually send — which you then edit, because it is a draft and it is wrong until you agree with it. Nothing is kept unless you keep it, and nothing is sent by writing it.
It gets more useful the second time, because it remembers what you did and asks afterwards whether it worked. Advice that never learns whether it helped is advice from someone who was not there.
If the conversation does happen and goes badly, there is a way through that too: you each describe it separately, and neither reads the other’s account until both are written.