Hey @Rusticanella sorry you've had to find yourself here, but you'll get great advice.
I'd suggest getting Eva Musby's book and looking on her website. She is a parent of someone with anorexia and her book and website are about one of the main treatment approaches, family based treatment (fbt).
Anorexia can really creep up on young people and families. My dd said the same things as yours - she felt full all the time, sick, sore tummy, etc. I did my best to encourage and support her to eat more but it didn't help, and probably just delayed us in starting to turn things around. Once someone is in anorexia, they very much are not able to change the patterns themselves, no matter the advice, straight talking from health professionals or scare tactics. Eating, restoring weight and health is the most essential part of the start of treatment, and often is a good indicator of eventual recovery, so usually it's best to start as soon as you know it's medically safe to.
The first phase of fbt removes choice and control from the person with anorexia. They are required to eat 3 meals (at least 500 calories) and 3 snacks (at least 200-300 calories) every day. They don't get to choose food, prepare or plate it, they have to finish everything and not leave even a small part. They are observed while eating and after (because they are highly likely to discard food). It's usually hard going, and causes distress to the person, as an anorexia person is terrified of eating. It's very distressing as a parent too. But it is necessary. They will probably shout, scream, cry, be physically and verbally aggressive, try to run away, withdraw. But despite this the bottom line is they must eat what is in front of them.
Good luck at the GP tomorrow. We were in your shoes almost a year ago now, and I remember it very vividly. We started 3 meals and 3 snacks that day, while we waited to be seen by the eating disorder service.