Last year we discovered that our DD (16) had an eating disorder. In the last 18 months she's been diagnosed with ASD too.
We (parents) had no experience with autism and it was not at all on our radar, despite DD once mentioning it in passing. But she never discussed it with us or asked for an assessment until it became an eating disorder and things ground to a halt.
We went on to the CAMHS waiting lists for everything (different "pathways" here in our health trust) and got to see people for her eating disorder first. That was a disaster for DD because she refused to engage, refused to admit she had an eating disorder. Their main focus was to monitor and increase her food intake - this involved us parents maintaining a food diary, encouraging her to eat 3 meals and 3 snacks each day (not an easy task by any means and not always successful each time). DD hated this, it took away her own control but CAMHS and common sense! made it quite clear that she couldn't continue and be/ become healthy on the 1 meal a day she'd restricted herself to. This only partially worked and DD hated being weighed and monitored.
However, some good things came out of it: anti-depressants were prescribed for her (suicidal, depression and anxiety. I'm not a fan of quick-fix medication but was desperate for anything that might help her), support for us parents and CAMHS encouraging us to pursue a private ASD assessment if we could afford it.
They've since signed her off their eating disorder service because there wasn't much they could do with her aside from the anti-depressants.
We managed to get a private autism assessment for her and she is having therapy following that. We (parents and DD) think the assessment and the therapy especially has been the most useful thing that's helped her, it has enabled her to understand herself better and (slowly) giving her the ability to talk to us about herself. I often feel as though I'm failing her with my own lack of ASD understanding - something I'm trying to rectify through reading books that DD recommends. It's a long and slow journey, for her and for us.
Like your DD, my DD often closes down and struggles to talk to us, she considers any discussion a confrontation, which she hates. She told me that she is more comfortable via writing, texts or letters because it gives her time to process and respond. It's not ideal, because you can just as easily misunderstand each other but it's a form of communication that she can cope with, so it works for us.
DD now usually eats 2 meals a day and various snacks in between, not super healthy (lots of crisps, biscuits and chocolates to keep the calories going in) and she is maintaining her weight and not losing. She now acknowledges that she has/ had an eating disorder, that it is her way of exerting some control over her life, she still struggles to eat the amount that she does.
It took 4 months from the GP's CAMHS referral to see someone in the eating disorder service, 16 months before she reached the top of the autism waiting list. If there's any way, I would try to find someone privately to help your DD.
In the meantime, whilst on waiting lists we were offered some help as parents, on how to support our DD - perhaps that might be available where you are? 