Teenagers are not fully formed adults. Their brains are not fully developed and certain logic making decision skills are not yet there. They are also having massive hormone drives, and at 16y many are going through the first high level stress they've ever experienced where mum (or dad) can't sort it for them.
She's had the move enforced on her (as have you) and it's out of her control. The reality is different perhaps to her expectation, smaller, less space, some more restrictions than before, etc. All understandable to adults, but still a negative in her eyes.
Teenagers often are probably more like over grown toddlers, than they were at 11/12.
But it will come in time, with support and guidance form parents and adults around them, some understanding and empathy and some realistic truths told to them.
Does she have her own money, so she can learn its value more and understand the whole saving and spending balance better?
As it's so close to GCSEs I would now focus on that room for her. Help her have a cosy safe space where she can get her head down to revise and rest. I don't mean spending ages getting it perfect but just making sure it is a good place to study in more than anything else.
GCSEs should be her main focus really as we are now only about a month away from them starting.
Tbh at this stage last year Dd wasn't really doing much socialising anyway as her free time was mainly revision.
Then maybe set a date up for after her last exams to go and sort out the bedroom properly - sell it as a reward for good effort during her exams maybe, if need be.
And although she'll be off to university is 2.5 years, many still come back home for holidays and more and more are returning afterwards as the housing market is so tough. So this could well remain her home for many more years to come - at least 5 years through school and university as it is.