@Teadrinker81 It’s so hard, isn’t it, especially when they get into a doom spiral with confidence and motivation going down the plughole together, and then somehow it is all your fault.
In your situation I think I might adopt the “good cop” role, since you’ve ensured that everything is in place to help her. So let her teachers and tutor chivvy her along, leaving your role as focusing on the positive. Does the tutor give you regular feedback? (They should). Then you can say - “Oh I hear you did really well on the statistics homework! Well done, that’s a great achievement”. (Or - she finished all her tasks before the deadline, or whatever good news story you can think of).
Even if you get a grumpy shrug, it will mean a lot to her that you are noticing and acknowledging her progress and her effort, and by showing confidence in her you will be helping her regain confidence in herself.
We went through a very difficult period with DD’s maths in Y10, but with a lot of support she turned it around in time for her GCSEs. I know A Level is a different kettle of fish, and hope you find a strategy that helps DD to feel better about it.