Also, mum has a tendency to start screaming and yelling at me when she finds out about mental health problems, which I'm sure you know is about the last thing that's needed.
I sympathize so much - I live with my mother, due to being disabled, and I'm nearly 30. She abused me as a child, she's still abusive, and she's just like that - yelling at me over my mental health issues, blaming me for them, blaming me for not being able to snap my fingers and just get instantly better.
I have no choice right now - I just have to sit and wait for disability - then I can get the f*ck out of here and sever all ties.
It causes so much anxiety for me. Severe anxiety every day - these sorts of situations are really difficult. I don't have much to offer, but, you're not alone in having to deal with the problems of living with abusers, while having PTSD from them and/or other things.
As a former teacher though - and someone who was abused during childhood - it really pisses me off that there are kids stuck in situations like yours.
Could you perhaps speak with a school counselor, or school psychologist?
They might have experience with many students who have trauma histories, and have resources to help you.
Edit:
In regards to helping things be more bearable while you're there - I would try to avoid the abusive people as much as you can. I know that can be hard when it's someone you live with - but if you can preoccupy yourself with other things, perhaps spend time in some kind of safe space away from your house, that you can think of - like a public library, university library, so on and so forth. If you're able to, and if it's safe.
Perhaps after-school activities can help you spend more time away from home - though I don't know if that would help your anxiety or not.
Just throwing suggestions out there.