IMO their mental health is more important than anything else. Certainly way more important than school. Please let her know this. Tell her that her priority and yours too is for her to have a happy, healthy life and to develop confidence and resilience to deal with the inevitable stresses. If she is truly overwhelmed, explain to her that nothing at all is set in stone. It's possible to drop out and start the syllabus again at a sixth form college from September if she prefers. Many highly successful people have low or no exams from their school days. It doesn't exclude you from doing a degree with the OU or an access course and a regular uni course later in life. In short: the whole world and anything she wants to do in it will still be available to her if she completely flunks these exams or drops out entirely.
I know everyone hates an armchair psychologist (with good reason) but from what you have said, I wonder if she has something like ADD. Complete inability to concentrate and self-regulate to study for exams; room a tip; symptoms of depression and anxiety; meltdowns.
In your position, I'd do this:
Give her a duvet day tomorrow. She is ill with stress and needs a break. If you can, take her out for a long gentle walk in the fresh air. If you're at work, make sure she's got a pleasant and easy plan for the day (lie in, watch a comedy rerun, get hair cut or similar) and check on her regularly by text or phone.
Over the weekend spend a whole day working on her room. Declutter it with her. Get her to do one aspect at a time (An ADD person - if she has it - won't know where to start without a very specific prompt, so you just say: pick up clothes first. Put dirty ones in a pile, clean ones on hangers. Then rubbish in bin, dirty crockery downstairs in sink, etc. Make it fun. Put on nice music and burn a scented candle if she likes them. Open the windows, change the bedding. Finish by cleaning it and ordering any storage she needs for keeping stuff organised.
Once it is orderly, check on her every night and give a specific single instruction to keep on top of it.
That may seem like a superficial thing to start with, but if she is overwhelmed, having practical proof that she can tackle a problem and resolve it will help.
Once the room is tidy, help her sort her school work. Go through her files with her, organising material into folders and sections, checking she has all the text books and revision guides she needs. (If she has ADD or similar she will have lost half of them and may be too ashamed or worried about getting into trouble to admit it.) Make a list (with no judgment) of all the things she needs but lacks - from supplies to missing chunks of work.
My niece is very like this (including the PMS) and my DSis just did nothing but 'endure' it. I was itching from the sidelines because I was like this too and would desperately have benefited from someone helping in the way I've just suggested. DS2 is like this but I do help him and as a result his anxiety and chaos are very well under control. he has learned how to organise himself far better than I ever did but needed to be taught every step as nothing came naturally.
She may need or want counselling but I'd first give practical help and comfort in as many areas as you can and get her life on track like that. Otherwise she'll think you are just reinforcing the message that there's something 'wrong' with her and she probably already feels that. She needs reassurance that she is fine as she is and can overcome this and thrive, even if she needs to take her own time about it, just as many many successful happy people before her have and will after her. We're not cookie-cutter dough.