Hey man - looks like you are lucky enough that we happen to be posting at the same time - but don't let that make you feel like you need to act fairly quickly on this. I often find that stuff I write or say in a fit of emotion is much more damaging and useless than if I write something down, sleep through the night, and wake up with a clear head and think about it. I nearly ALWAYS completely re-write my first thing at that point. In fact I use that first opportunity to let all my emotions out into my writing. Then the next day I can look at it on a higher level, and consider what really matters.
Even if the only option at this point is breaking up - you want a GOOD break up. You want something that you can look back on that will empower you, for being the better person, for improving yourself. Not yet another situation where you are thinking "Ah geez if I had have just waited a bit, I could have said this, instead of this, argh..."
I can't tell you exactly what to do, you kind of have to feel the vibe, because of course you are the best person to make the final choice, actually being in the situation.
But I would suggest freezing out is a more subtle thing you can do early on, when she's actually capable of feeling guilty for her actions, and it looks like she's losing you. At this point it doesn't look like she's caring too much about that. Maybe you could step up to freezing first, just to try it, if you've never done it with her - see what she does. But my first thought is that, that takes time, and might look like you're just being grumpy, instead of losing interest, because what she's doing is just too bloody obviously disrespectful. Again, think on it. But think clearly and high level, and wait till the next day if you need a chance to clear your head.
If you go for confrontation - don't do it through electronics
Be a man and talk to her face to face. You are not a person to hide behind text and find it easier to "open up" in writing. And if you are, then you need the face to face experience anyway. Take your time to think about what you'll say and then when you do, speak from the heart, don't sound rehearsed. Hold in your mind what is so wrong about what she's doing and go from there. You will find some of the words you thought of earlier as you go, and it will sound more natural.And that message that you came up with, based on what I said - as long as it really speaks what you feel needs to be said, and succintly, then it is good. I wouldn't add "you will lose the best thing that ever happened to you" because that's an outrageous claim and is obviously fabricated to make her feel guilty. Just boil the situation down to its essential facts - eg. that part should really say something like "you will lose me."