From my experience, as a previous TA for teenagers and a close friend who works and lives with challenging behaviour the support for you is just as important as for the child. Sadly, I think the 'gap in the market' hasn't been filled yet with easily accessible support.
At school TA's and teachers get support, time to sound off, or take a break, talk over feelings, express thoughts like you 'let the child down'. Generally debriefing and it helps enormously in gaining perspective and reclaiming personal power. Parents need similar but its harder to get.
When the tipping point happens its almost impossible to remain in a grounded, strong place which is exactly what the child and the adult need in order to 'contain' and manage the behaviours and emotions that can come thick and fast and burn themselves out equally quickly.
Sometimes I would deliberately 'switch off', detach almost, remain present to the goings on, stick with clear instruction and guidance whilst acknowledging the behaviour but not giving it attention. Sort of helping them out of it, if it was safe, I'd just let things run their course.
To do all this is exhausting at times and conserving energy and choosing how to do that is something that helps I think. Is there a way you can get support for you? Some self compassion, a list of essential things you want to improve? Like the getting ready for school routine. Take on small goals.
11 must be hormonal too. Challenging.