Denial and threatening with a knife

Maybe not suicidal but as someone who was a teen in a similar situation some 17 years ago I will say this looks very strongly like depression all the same, depression mixed with meltdowns often look like this it's an explosion of things we aren't expressing in our daily life and it gets too much. What she really means by threatening to kill herself is she feels bad and wants the feeling bad to stop.     (Disclaimer, this next is mostly guess work based on my own experiences but bear with me until the end.)
She probably doesn't have great self esteem because she's internalised all that missed school time as a huge failure and feels like there is no point in trying if her past experience so far has taiught her she will not succeeed anyway. Maybe she needs some kind of therapy that focuses on things like Learned Helplessness because it sounds like she has gone to ground and rooted herself in a limited routine of her safe things. (Or at least perceived safe things because the social media doom scrolling specifically is not likely to be helping.) Her TV sounds fine, she probably has a handful of comfort shows she's bingeing over and over? (feel free to tell me if any of this sounds right or not, The way my Autism works is I just have to understand things through a relational/comparitive lens first before I can reflect and give actual advice.)

Thank you Sam. What you have said in both sections really chimes with whatI am seeing in my daughter. Yes she binge watches TV shows (Gilmore Girls, Stranger Things etc) for comfort. I have never heard of Learned Helplessness but would like to. She has been resistant to a therapist so far, and we have been advised not to get her to go unless she also chooses to go or it will backfire on her seeking help in the future. But she does need help. I really appreciate what you are sharing of your own experience here. If you have more to add or need more info, please say so.

I agree you can't force her as that maybe traumatic and add to the issue, especially since she'll already be hyper sensitive to feeling as if she "has something wrong" with her. I will say it's not the end of the world if she misses GCSE's in secondary. Actually I'm of the opinion that academically we shouldn't be making kids do GCSEs at 15-17 yrs old because those are the peak hormonal years where we aren't kids anymore but we don't know how to be adults yet either and it's a huge ask for stable brain chemistry when we are at an age where our whole body chemisty is a mess. I think if you can take the pressure off and just reassure her you love her even if she needs time that is the best thing because you can retake GCSEs as a mature student, it's never too late to start.
I would have liked to retake mine at 20 but I was made homeless and not in a physically stable place and thus still not a mentally stable place for a long time, and I also had to break out of the learned helplessness which was incredibly difficult, if it weren't for financial and emotiona support from my Husband I might still be stuck. But I was able to retake my GCSE Eng and Maths at 27, have a few more rocky years and weather out the pandemic and then last year I got my BTEC lvl 3 and This year I have started my first year of University, 10 years later than I would like haha but I'm here. And hopefully with that emotional support and breathing space for her to come out of it on her own terms your daughter will also be able to find that courage and self esteem to start again... and not as late as me. Gosh I wish my Dad had been half as sympathetic as you seem to be, I might have been a bit quicker at restarting after my hormones settled otherwise.
I also wanna add that in colleges they are generally a lot better with helping students on the spectrum in my experience, and you can ask for extra time in exams too.

Sorry I took a while to respond again. I think I know how she feels, sometimes I get contrary and it's just because I'm stuck in NO mode, sometimes it's like we would have come around to something if given enough time but because we've been prompted to do so before we're ready we turn into that Pingu meme. I don't even think it's always purely emotion fed obstinance, sometimes it really is because the second someone else suggests, implies or outright states we should do something it feels like it has removed our agency of having chosen to do so for ourselves adding to a sense of fustration or deflation.
https://i.imgflip.com/3tezvo.jpg (Pingu meme example.)
Definitely try keeping to phrasings of could do rather than should do whenever you can and leave these things as totally up to her that way she can come to them of her own accord.

I wish you the best of luck and hope she can get through this with you.

She sounds creative.

Perhaps channel that in her. 

Sam, you sound really like you get my daughter! I try to find things to praise. Like she has been helping me with Christmas shopping and is great at it. She repainted her room last year and was good at it. I think some household chores might help her feel valued in the household. But she strongly resists he making any other suggestions about what she might do with her time, even when it's stuff she likes to do.

I really feel my daughter will come to herself and be very talented at something the future too.

Thanks that is very useful insight. It is difficult as a parent to judge when to take the pressure off (and risk facilitating dependency) and when to give a bit of tough love so that she finds her own motivation. Since December 2021 she has completed 5 weeks of school, which was Sept/Oct this year. I don't mind giving up on school altogether - it isn't for everyone, right? But it is really hard to say 'OK just watch TV until you are ready to do your GCSEs in 5 years.' Filling the void that school leaves is tricky when everything I suggest is rejected (probably because of hormones), and social media algorithms are designed to keep kids her age hooked.

That's awsome, congrats to him for doing it. :)

My brother was the same; rebuilding his life, via Tech. He graduated in 2020; a few marks shy of a First. 

I also want to add that when it comes to getting stuck in an unhealthy routine like doom scrolling through social media one of the best things is not to simply rip that routine away because it's become such a crutch for her stability but if it could be replaced by a healthier crutch (maybe a new hobby somewhat connected to a preexisting interest) with say, an extra reward incentive process that could be helpful motivation too as when feeling depressed the low energy often manifests as low motivation to do anything outside the current routine parameters.
I realise my experience is just my experience even if others are simililar they won't be the exact same, but with learned helplessness I also found it good to retroactively reflect on past sucesses and finally get around to celebrating and rewarding myself for those, and then rewarding myself proportionally for every new sucess. Even just the little victories that broke me out of the slump, like putting laundry in the machine this week for example. Because sometimes we need to build up those little successes so we can feel capable of larger success.

I also want to add that when it comes to getting stuck in an unhealthy routine like doom scrolling through social media one of the best things is not to simply rip that routine away because it's become such a crutch for her stability but if it could be replaced by a healthier crutch (maybe a new hobby somewhat connected to a preexisting interest) with say, an extra reward incentive process that could be helpful motivation too as when feeling depressed the low energy often manifests as low motivation to do anything outside the current routine parameters.
I realise my experience is just my experience even if others are simililar they won't be the exact same, but with learned helplessness I also found it good to retroactively reflect on past sucesses and finally get around to celebrating and rewarding myself for those, and then rewarding myself proportionally for every new sucess. Even just the little victories that broke me out of the slump, like putting laundry in the machine this week for example. Because sometimes we need to build up those little successes so we can feel capable of larger success.

Sorry I took a while to respond again. I think I know how she feels, sometimes I get contrary and it's just because I'm stuck in NO mode, sometimes it's like we would have come around to something if given enough time but because we've been prompted to do so before we're ready we turn into that Pingu meme. I don't even think it's always purely emotion fed obstinance, sometimes it really is because the second someone else suggests, implies or outright states we should do something it feels like it has removed our agency of having chosen to do so for ourselves adding to a sense of fustration or deflation.
https://i.imgflip.com/3tezvo.jpg (Pingu meme example.)
Definitely try keeping to phrasings of could do rather than should do whenever you can and leave these things as totally up to her that way she can come to them of her own accord.

I wish you the best of luck and hope she can get through this with you.

She sounds creative.

Perhaps channel that in her. 

Sam, you sound really like you get my daughter! I try to find things to praise. Like she has been helping me with Christmas shopping and is great at it. She repainted her room last year and was good at it. I think some household chores might help her feel valued in the household. But she strongly resists he making any other suggestions about what she might do with her time, even when it's stuff she likes to do.