Op, I really sympathise, I could have written your post, my 17 year old teen is exactly the same, except I suggested and arranged some counselling and she only went three times as she said it wasn't helping. Lockdown has taken it out of her too. And I think it's been especially hard for sixth formers and university students.
If your dd is asking for help I would definitely try and get it for her. I don't think most teens find it easy to ask for help as a rule. Can you dig down and find out what specifically is worrying her? Sometimes just the reassurance from someone else other than their parent that what they are experiencing is "normal" or at least fairly common, can be helpful.
One of the reasons my mental health was shot to so much shit in my teens and beyond was because my parents decided I was a normal teenager for as long as it suited their narrative and preference for me to be, but a terrible monster when they wanted to make out how dreadful I made their lives
I'm really sorry you had such a dreadful time as a teen DrSbaitso. I don't know your circumstances and wouldn't presume to comment on them. But if I may speak in very general terms, from my current individual perspective as the mother of teens, , I don't think what you have written above is as contradictory as you might think. I love my DD so much, I am very worried about her, at the same time I think she is a pretty normal teen and she is really putting us through the mill atm. She can be delightful one minute and vile the next, just as the op describes. I think it is possible to be both when you are a "normal" emotionally labile adolescent.
Obviously some teens may need more help more help than others and if a teen asks for help them I would definitely try and help them to get the support they need.
Again, not commenting on your individual circumstances DrSbaitsobut but I was very relieved to see the op's comments on this thread as it is almost word for word what my DD says, about hating our decoration and wanting to move out and live on her own. She also "hates" me, doesn't hesitate to point out my failings every day, and hates almost every aspect of her home life. It is quite hurtful and I have been desperately worried these past few years that it was my individual parenting that was causing this issue and that I had completely failed as a parent. (I'm not saying I am a perfect parent by any means, I have made lots of mistakes, but I have always put my DC first above anything else and tried really hard for them.) But threads like this remind me that it is, to a degree "normal" for a teen DD to become disenchanted with family life and want to strike out on her own. And these emotional ups and downs are nature's way of cutting a teen girl off from her family and preparing her to become an independent adult. It's a very hard process to go through sometimes . It's very hard for the adolescent themselves, and sorry, but it can be very hard for their parents too. Part of me knows that DD doesn't realise how difficult and wearying her behaviour is and that is what is getting me through atm but honestly, the tension and emotional upset every day is taking a toll on my own mh.