My Yr12 daughter, could be the twin of yours. Bright and articulate but apparently unable to turn that it into the written word, and has the GSCE results to prove it :(
After years of trying to deal with, I have come to the conclusion that the root cause is a complete lack of confidence in her own intelligence - whatever she thinks of cannot possibly be what they are looking for, there must be more. So she ties herself in knots trying to think harder and ends up putting nothing down on paper, and then panics when the essay has to be in the next day. And of course panicking feeds into the spiral of 'uselessness'.
It is also obvious that English/history essays, where you have to express opinions are far harder for her than the fact-based sciences. But she must prefers those subjects - reads constantly.
Fortunately, I have been able to help her a little with the English, with lots of tears and frustration, by talking to her about the questions - just getting her to tell me what she thinks it means, making notes from that and then sorting the notes into themes - so for example write a paragraph on the different use of language, with quotes that explicitly refers to the title of the essay. Then another on structure. Then when she has three or four, look to see how they can be linked, so it's not a series of bullet points.
It seems to have helped, and a couple of better grades recently has proved that what she thinks is good enough.
So at the end of that particular essay :) my advice is to find a way to convince her that what she thinks is good enough, And
that you, her teachers, a teacher sit her down and explain properly and often enough to sink in, how to write essays. It really isn't innate magic - there are rules you can follow which may not get you straight As but ought to get you the A levels you want/need.
And btw, my 'disorganised, uninterested' DD is far less so, now she's beginning to feel that she can do this, that she's not a "thicko, who's going to fail, so what's the point."
Phew, really hope that helps :o