How do I deal with mentor making my life harder than it needs to be?

I am currently on a SCITT course and have to teach 80%. The school starts at 8.30 and finishes at 3.15. I'm at school from 7.15 and don't leave most days until 6. My mentor is horrible and nitpicks on things she doesn't do herself. She keeps extending my targets because she hasn't seen enough. It's not my fault that she only works 2 days and is out of the class when she's doing her science lead work, leaving me alone with the kids and no TA. I regret doing the course as it has affected my mental health so much. I feel that other staff in the school are lovely, and my mentor wants to keep me at the school longer so she doesn't have to do the teaching when she's out of class and has a free TA.

You seem to have raised a number of concerns. Teacher training is tough. You receive so much "negative" feedback it can be difficult to know what to focus on. But equally you might not know this but you are in control. You should have one formal minuted meeting per week where your targets are set. Make absolutely sure you know what those targets are and how to achieve them. It is your mentors job to ensure you know how to achieve them. Similarly, you should have one formal observation per week and this is where you demonstrate that you have hit that target.
In some ways being unsupervised is a brilliant opportunity for you to try things out without someone tutting in the corner. However it is really important that you reflect carefully on what went well and what did not and more importantly, why it went well or why it did not.
Keep going!

Hey my mentor is very critical and only likes lessons if its in her style! Our mentor meetings are her putting me down and making me feel bad about myself. I have ADHD & dyslexsia so I have DSA mental health person who has told me not to say anything I don't have evidence for.

Right - so it is important that you push back and identify specifically what she doesn't like and what you need to do to put it wrong. You need to ensure she structures her answers so that you can achieve.
I am not sure what ADHD or dyslexia has to do with anything and we all have mental health. The key is how we manage ourselves and those we interact with in a world that does not necessarily adapt to our every whim at each turn.

The headteacher has watched a whole afternoon of my teaching and praised it I’ve passed all my targets so I don’t understand why they still want me there…