Mini, here's my opinion fwiw... :)
Don't stop posting. You need the support you can find here. As others have said, MN has seen me through some really bleak and difficult moments over the past couple of years. Even if you do have your own counsellor, s/he will only be available at set times (generally once a week on a weekday), not at the moments you really need support (like weekends, and 3am, when everything seems less manageable).
Don't tell her you're going to stop/lie to her about it. I can think of loads of reasons why not... Honesty is important. Presumably you don't want her lying to you. Your relationships needs more trust, not less. You are effectively being controlled by her if you do. You will be tied into more lies if you do. The fall-out if you are discovered after a bare-faced lie will be huge. Posting here is not wrong; it is reasonable and useful. She needs to accept you have a right to access support too...
However, you need to improve the steps you take to protect her anonymity. Name change again. Add a password to your a/c - there is no way she should be to access your PC or your MN account.
Reduce the number of identifying details you include. For example, don't say how old your mum is, or mention her DB in the same posts, or mention any details that aren't directly relevant to the issue your seeking advice on... Perhaps post about different issues under different nicknames. I agree with triac that you may well be identifiable to people who know you. I read your comments about having no secrets - "There is nothing I have put on MN about my family which is not already known in my community, or which I'd be mortified at someone finding out". That is perfectly healthy, but it is clearly not a feeling your DD shares. Out of respect for her, I think you should be more circumspect. I agree with wannaBe about the public nature of online forum, and as a rule of thumb, I'd say be more careful about what you say and how you identify yourself and your DC here than you would be talking to a stranger at a bus-stop.
Be aware that what she is doing is probably (at least partly) an attempt at 'distraction techniques'. My DS used to do it all the time: he's be in trouble for X, and when I challenged him, rather than deal with that, he'd respond with something along the lines of "Well ANYWAY, I don't need to take responsibility for X because YOU did Y yesterday".
You need to be strong and focused, and keep saying things like "We're not talking about my Y now, we're talking about your X. If you want to talk about Y too, we can do that later, when we've sorted X".
I strongly agree with what Maryz and other say about shifting your attention onto yourself. You have probably spent so much time thinking about your DD that you have forgotten to look out for your own needs. And I always think: if I have forgotten that I am worth looking after and respecting, then no wonder my DC have forgotten too. You need to be clear with yourself as well as your DD and others that you have a right to respect, safety and support. :)