Op I have to go out so forgive brief response.
Please know that you are not alone.
Teens go through enormous plasticity in their brains, just like a toddler does, and the onslaught of hormones can make them very difficult to live with.
Obviously it’s hard to know what is going on and why your middle teen is reacting this way but seek out a licensed psychologist experienced with adolescents.
Look up the organisation PEGS UK that deals with children on parent violence which happens more than you think.
Don’t bother with the Mumsnet search engine but Google “Holding on to the end of the rope Mumsnet” and you will come across four threads on the Mumsnet teen board that discusses the same issues you are having: read at least the first two,
Lastly: Google Lisa Damour and listen to her podcast and read her books!
In general, try to take a step back emotionally,
Do not whatever you do take their emotional crises personally. They are lashing out at you to relieve themselves of their own anxieties.
Spend some time boosting your own confidence and self worth by doing things you enjoy.
Live your own life and although it’s very hard to do, try and model positivity and cheerfulness to your teens if you can.
They need us to be the calm granite rock that holds them steady while they thrash about a bit!
And they also need us to show them that adulthood is worth striving for!
Don’t get so entwined with their emotional ups and downs that you get overwhelmed too. And if you feel yourself going that way; get yourself some support too,
Adolescence is a turbulent time when some mh issues can manifest themselves so keep an eye on that, but in most cases, this will get better, they will come out the other side but it’s a marathon not a sprint. So treat yourself well and bolster yourself up as much as you can. Don’t allow yourself to be drained of all emotional resources because you won’t be able to help them then.
Good luck 💐