What subjects is she doing @Perfectlystill?
Staying absolutely on top of them is really important I think, so catching up anything missed straight away, if something is hard to understand, work on it until it isn't. Y13 is pretty tough and Jan - May is so busy trying to get all the content finished while also making revision materials, learning stuff and doing past papers, there's not much time to faff about! Start preparing for exams nice and early so that she can have plenty of time away from studying too. DD put off her revision for so long that she ended up with a ridiculously tough revision schedule just before and during exams, and that was so hard, I think she's still recovering.
Your job is to be very calm, absorb any stressy energy, keep reminding her she can do it and why she's doing it. Uni seems so far off in the dark days of February! Especially if she's a bit lazy, you might need a little conversation about whether or not she really wants to give herself the best chance of getting to Exeter and what that means in practice - revision and exam prep. Ask her how you can help, whether that's by making cups of tea, waking her up, testing her, keeping her accountable, keeping up supplies of special foods, just depends what she wants and needs from you. I made a lot of nice breakfasts 😂.
DD has a tendency to dig her heels in if she thinks I am nagging or trying to get her to do something, so I had to be clear that I really didn't care how hard she worked, her exams, her life, but I was on the team if she needed me. (She does have ADHD and struggles with focus and procrastination so I was probably more needed than most.)
There's such a lot that they can't control, anything can happen in exams, so I think it can be useful to reframe the goal away from grades and towards how do they want to feel going into the exam room - prepared and confident that they're going to do well, or scared and dreading it. And how do they want to feel when results come out, especially if they do drop a grade or two - knowing that they did their best or regretting that they didn't do that extra few hours of work.
Here endeth the lesson 😂