Yes this also sounds similar to my DD - she’s now 18 but EBSA and all of what you describe when she was 13/14 when the wheels started coming off. of course no one on here can make a diagnosis but there are enough posters on here who have gone through similar to say it’s highly likely.
DD got worse in her room and was soon very depressed (but like your DD would still see friends infrequently- and seem happy in that moment but as you say - it was definitely masking) . No school for the best part of 2 3 years in total.
We did her somewhere with CAMHS as there was lots of self harm and attempts on her life. And she received a diagnosis of ADD and also Autism.
The depression and subsequent burnout come from the exhaustion of trying to conform to school, social norms, masking and feeling very different to those around them. I didn’t realise it was autistic burn out that she had at the time until her diagnosis and living through it / reading a lot about it.
Even without a diagnosis I would ‘assume’ that she is in your head at least amd so as much reading as you can about it. ‘Autistic girls’ network’ Facebook group was a brilliant source of information and comfort for me.
I thought similarly to you - ie how can this only be coming to light now? But as others have said - it’s often exactly that this age that trying to cope (as life / school etc gets more tough). However now I look back there were quite a few things that didn’t seem anything other than normal at the time in primary but actually the signs were there when I read up and really thought about it.
My DD is now 18. Somehow she made it through GCSEs but dropped out of A levels after 6 months. The pressure and school environment were just too much. She spent another period of depression but has now got a bar job . She’s à completely different person now. Happy, confident, loves her job and now knows herself much better. Maturity is a big part of it. I hope she returns to education at some point but for now it’s just great to see her enjoying life again.
As someone else said -I would advise low demand for now and lots of kindness and patience. Traditional teen parenting doesn’t work in these situations. It might take a while.
Also if you can’t get anywhere with right to choose or CAMHS and you think there may be any element of ADHD there (as they often go hand in hand) then that’s the one I would consider getting a private diagnosis for if that’s an option for you - as someone else has said - the medication for that can result help. It acts as a kind of antidepressant for DD as well as other things. They work much better than antidepressants do.
I do feel for you. I would definitely try and find (on line is great) a couple of private FB groups to join. Also - DD tended to find friends who had similar issues and it was really useful to her to know and talk to their mums as it’s so stressful and isolating for a parent to go through.
Good luck