She's obviously got completely unnerved at being alone. She sounds like she has developed a pathological fear of something bad happening to her when she's alone. This could be a deep-seated response to the shock and fear of Covid. It could have become ingrained, and has been made worse by hormonal changes.
To me, there is only one answer. I would go back to a routine that worked when she was younger. And keep doing that for as long as it takes, until she naturally relaxes and gains confidence again on her own.
I would not stress her about it or make any kind of fuss about it. I would try to play it down completely and say aw don't worry, sure you won't be wanting to sleep with us when you're 16 : )
The options I'd see are:
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Remove the pressure entirely. Keep her with you at all time. Don't ever ask her to go upstairs on her own, or go to sleep on her own.
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Agree a bedtime routine with her. Take her up for a bath and do something in your own room so you're around. Then say hey it's nice anyhow to read you a story or whatever at bedtime. Lie with her. Maybe read or tell a story, or she reads and you read your own book, but have that wind-down time. Then when she's fallen asleep, leave her.
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If/when she wakes in the night, tell her rather than calling out, just to come and sleep in with you.
I KNOW this is big commitment. And might feel like an alarming regression at her age. But she clearly needs this reassurance at the moment. I don't see how else this will get sorted.
If there's no pressure, and she can feel 100% secure and sure that it's ok to be with you, she will at some point relax. And then she will probably at first wake less in the night. And over time, she will end up just staying in her room.
If you literally put her to bed, then at least you can plan around that evening commitment.
And if she just slips in next to you in the night, you shouldn't have a broken night.
It doesn't matter if she needs this now, and she sleeps next to you. She's frightened. She needs you. It's ok, and good, to be doing the right thing for her. The only way to help her is to be with her, and not make her feel bad or anxious about her anxiety itself.
I personally think that if you change your mindset and do as above, it will be the quickest way to sorting it.
I know it's a shift in terms of your own relationship with her father, but both of you need to face this solution and just do it. The sooner the better.
Honestly she won't be in with you when she's 16. But she needs this now. She's fallen off the bike. Needs picking up and putting back on, but she needs to get her confidence back.