It can't be a threat of an ultimatum, it has to be you protecting you.
Yes, read the book, )look at your library) even knowing that you're working toward something can give you the calm to not act out of emotion.
You can start your boundary making by experimenting, writing things down, "H, as long as you choose to be in a R with OW..." be clear, clean, concise. You can tell him how you feel but choose only one or two descriptive words.
(( ))
Con't make a boundary you can't or won't protect.
Me 57/H 58 M36 S 2.5yrs R 12/13
Let me give up the need to know why things happen as they do. I will never know and constant wondering is constant suffering. Caroline Myss