It feels unfair and kind of personal - however much Pp are telling me that cutting annoying clients off without warning is within the professional remit of how a therapist operates.
It is unfair OP. Endings, however they come about should be handled really thoughtfully. Cutting annoying clients off is most definitely NOT within the remit. It’s damaging and doing harm to end a therapeutic relationship in the way you have described.
If you were abusive to her then that would be a reason to stop working with someone, but that would need to be stated very clearly and in a kind and thoughtful way.
It is NEVER ok to blame a clients overwhelm.
It sounds like you have a really big day ahead of you tomorrow and you have a huge amount on your plate. I’m not sure if this would fit for you and how you tick but I have a suggestion you can take or leave.
How about writing down three things you want to say to her (so you don’t spend ages on it - may be set yourself a time limit - it doesn’t have to be complete or perfect) put it in an envelope with her name on it and then place it somewhere to post if you still feel the need in a couple of weeks. You can then may be feel like you’ve communicated today enough to put it to one side and focus on looking after yourself tonight.
What helps to feel calm? Distraction? Relaxation? Bath? Book? Music? Chat with a friend? Cleaning? Whatever you feel you need and is possible do that now. Can you take a moment to put together a self soothing pack for tomorrow? Think of your five senses. A small teddy, a nice smelling hand cream or room mist, a soothing play list etc.
Life is sending you some really tough stuff. Be kind to yourself as much as you can.
Sending you care and comfort 
Please MNers no more telling the OP where she went wrong. At this point that is NOT helpful. It is not the clients responsibility to set boundaries and a good therapist can handle far worse in a professional and compassionate manner.