I went through most of your situation with my DD who's now 17. As far as I know she didn't have any bullying issues at school, but went through 18 months of anxiety, school avoidance, isolating herself, etc. Her mood swings were terrible, and then she'd be a sobbing wreck. She targeted most of her venom towards me, but I was always the one she called/ texted when she needed someone.
I'm a fixer too; I just want my kids to be happy; I also have a history of anxiety! I'm sure a lot of my family and friends are thinking, "she's indulging her", she's overprotective," "DD's nearly an adult", etc; whilst all I think is "you don't know her like I do", "she's young and vulnerable and needs protecting!"
But... now she's a little older I have loosened my reins, learned (slowly) to ignore and detach from strops and demands, and let her make her own mistakes. She goes to a Chinese therapy clinic once a month for massage, acupuncture, tapping, etc to relieve anxiety, which has helped massively. It's £40 a session but worth every penny! She's also been referred, through GP, for CBT and AS assessment (that'll take months to get an appt!!)
She drives me mad; she's lazy, messy, unreliable, self-absorbed and often grumpy, which is fairly normal behaviour for teens, but I can see a chink of light, now that I've backed off a little, and she's beginning to shine as an individual!