OP, think of a tree, grown inside the controlled environment, where no harm comes to it.
It sadly provides a powerful cautionary tale about the dangers of overprotection. In a controlled, windless biosphere, (like the Eden project for example) trees are shielded from the harsh forces of nature. They receive optimal water, perfect nutrients, and constant warmth, allowing them to grow rapidly and look pristine. However, because they never battle the wind, they fail to undergo thigmomorphogenesis—the biological process where mechanical stress triggers the growth of deep roots and thick, supportive "stress wood." Without this adversity, these seemingly perfect trees remain structurally weak and often collapse under their own weight.
Your DD needs to re-enter the world.
If she struggles with anxiety, she needs professional help, in the form of talking therapies, like CBT and support with medication to calm the mind loops that contribute to anxiety, for instance either Escitalopram or Sertraline are good first line anti-anxiety medications. They're called anti-depressants but at lower doses they control anxiety often very well. She needs to learn to deal with her anxiety so she can go out and live her life, like everyone else.
These medications can be life-changing. I'm fed up of SSRIs getting a bad rap. If it means they allow people like your DD to function, let them take one every day for the rest of their lives. Some people just have more sensitive nervous systems and the sooner we accept, the more people can be helped and get on with living a full and happier life.
Once her anxiety is controlled, she needs to go to college asap or vice versa. Try college and see how her anxiety copes, and then get the support you need.
You need professional help at this point. YOU need to reach out for help yourself because you can't manage this any longer by yourself. It's understandable to want to protect your child as much as possible but at some point, the pendulum can swing to far and I think in your situation this may have happened.
You did with the best of intentions but now you need to find another way to re-intergrate your DD into normal life. Talk to colleges and find out what the options are. More than get her into the world so she can start to live.
She's obviously a capable girl, done her Gold D of E except the expedition - that can still be done later when she's more fit and there's a better fit of people, now she needs some academic support to raise her general level to that same as the average person in the UK. I think that's possible. Come on Mum, you can do it and so can she.