Looking back at some of the things I learned the "hard way," one of the most important is say what you mean and mean what you say (but don't say it mean).
In other words, before you have "the talk" with her, have one with yourself. Determine what your boundaries are, and what you are willing and able to do to enforce them. Don't threaten to leave if you are not prepared to leave. Don't threaten divorce unless you are fully prepared to file. These are just examples, but it's very important to make sure you are clear with yourself before you can be clear with her. If you aren't, you lose your credibility and make things worse in the long run. (That's what happened to me.)
L