Seek- I am sorry you are hurting so much and it sounds like the combination of childhood/FOO trauma and your current r/s with your daughter create a terrible number of sad feelings (understandably).
Your description of your daughters response to your letter is, well, what I'd expect a response from an addict to be. She doesn't want to face herself so she is of course going to find any and all things she can to throw at you-- deflection, projection, denial, guilt trips.... And like any good co-dependent you are caught up in worrying about what your daughter is feeling/thinking/saying about you (I think this is something most mom's would feel, codie issues or not).
I don't have any suggestions/advice for you other than to say that you aren't going to be able to save your daughter from herself no matter how hard you try. I imagine that you hoped that you expressing (bluntly no less) how concerned you were about her might shock her into seeing how bad off she is and be receptive to help? Unfortunately I have found that the more I express my concerns to my AH (or did in the past as I am not doing so anymore) the more fuel it gave him to deny, blame and distort reality. And then I was the one who was upset.
I hope that you have some friends you can talk to? Al anon maybe? Sending warm thoughts your way.