There is no reason to believe that that is the "best you can do" with regards to a spouse. I doubt she herself believes that, and if she does she is quite the risk taker, isn't she.
Hang in there and keep working it thru in your mind until you find a way to present it to her that is so clear and concise that even she can't twist it to suit her needs. Whether that is in writing or in person, who cares. Just as long as you get through to her, and the sooner the better.
From my experience, you will likely have to say it in a way that sounds mean, or "feels" mean. Any other way and your message will be confusing and diluted. She will be thinking, "There's all this flowery stuff but then a line about ending the marriage if it doesn't change. Hmph, I guess he thinks he will threaten me..." She won't see it for the truth if it is disguised with too much "I love you but.." statements. Stick with what you know to be true about yourself and go light on the mush. When she has heard your true message, then you can incorporate more of the mush back in. I think it is hard to wrap your mind around the concept of "I love you with all my heart but I will leave you nonetheless if things don't change." Something seems incongruous about that statement but it's the truth, huh?
