CE - I would take some time and think about what it is you want from her before sending your letter. i.e. do you want another face to face meeting, or just to tell her what you think? Then write a letter targeted to the kind of response you want.

Sometimes when a person gets something emotionally laden they cant see the logic and only see that you are emotional. I think you have some very clear points to make to her, and I would make sure not to drown those out so that she can clearly hear them. I would be very concise and to the point, which forces her to address the specific issues and not a huge vague impasse issue which is too undefined to surpass. Perhaps even send two separate responses...

For example, I would tell her how much not smiling at you hurt and confused you, and felt abusive. I would ask for a better explanation of that, and then tell her that it is something that you are still not ok with. That while she might see it as an ultimatum, from your perspective you are just practicing self preservation... something along those lines. I guess what I'm saying is I would make sure it's super clear that is the issue, and force her to either address or ignore that.

I'm not a shrink, but I am really thinking the lack of feeling closure on your end is because she treated you like crap and refuses to see her role in your dissatisfaction. Either she's dumb or she's missing the point. I'm guessing now she is missing the point and this could be one big miscommunication. If you force her to look at the facts then she'll either have to ignore you or respond, and I think that might bring some closure... rather than her vague responses to what she thinks you are thinking and feeling. Not sure if I'm making sense, but basically I think you need to be really concise in how you approach this, leave the emotion out of it but discuss the feelings.