I haven't I'm afraid but can empathise as my DD (16 about to turn 17) was fast tracked to CAMHS after taking an overdose last year. She refused to engage with the service at all (which i have to say was poor with only 3 appts given; others cancelled by CAMHS at the last minute (that really annoyed her and me too as we'd literally be en route to appt and then get a text cancelling)
All stress in dds case stemmed from school. Luckily she was able to leave in May this year and she has largely returned to normal.
I think my dd might be autistic spectrum (it is in our family) and i am going to get a private asst (which she has agreed to)
I think last year she had autistic burnout but it was very frightening as, in the weeks after the OD, she was staring ahead and there was no light in her eyes at all. I thought she was in a catatonic state at one point.
Has your DD got plans for the future and is she happy with them?
Could she come out of education just now and out of the pressure? This is what helped my dd.
She really thought she had to go to uni! This did not come from me or her dad. Schools apply so much pressure these days onto brighter kids.
I think our teens shut down when the stress is too much and haven't got the maturity to try to engage with therapy. To engage with therapy, i think (and i'm no expert on this) that you need to be able to confront your own behaviours and thoughts - and that can be painful and difficult.
And it takes maturity to be able to look at yourself and talk about yourself.
So - as you say she acts younger, maybe the way forward is just to reduce pressure, give space and try to connect and spend time in a very non confrontational way.
One thing we did with dd was take her to a climbing centre. Just as a one off. I had read that when you do something like that, all your mind can focus on is the climbing so it is a break from worries just for an hour or so. She loved that!
Walking the dogs in the countryside.
Going to a cafe
Maybe go out on a boating lake or something. Dont talk about school or anything. Just enjoy being together.
Do easy stuff though. I thought baking together would be enjoyable for my dd but it was stressful as she was in such poor mental health, she could not succeed at it. It was just too overwhelming a task for her.
Hope my post helps in some way - even to know you're not alone