My eldest DD was Bulimic from abt 17, just before she went to Uni. I had not a clue! She presented normal at home - but I was blind to lots of things. She was often eating 3-34bags of crisps and bars of chocolate at a time, and taking them to her bedroom. Other foods as well disappeared. We just assumed she was passing them to friends or taking them to school for lunchbreaks. One day after starting Uni I was working in my study when she and her younger sister appeared at the door, and the younger one said "Mum, DD has something to tell you" and out it all came. I was flabbergasted, confused, not a clue how to respond at all. Fortunately I did not scoff or minimise the situation, nor say stupid things like "just stop doing it". I went to my GP and (despite still being my favourite GP) he said "the NHS doesn't have many resources for people who damage themselves deliberately"!
Basically, she was stuffing huge amounts of rubbish into herself and spewing it up later.
So I found a private consultant and she agreed to come. She complained all the way there that it was a waste of time, said "stop the car cos I am not going", but we did see the man, both separately and together. He was great, to my mind. He gave her little tasks to do that weren't necessarily to do with eating or purging, but she seemed to engage with him.
Unfortunately after two appointments she had to go back to Uni and in Lancaster there were no private Eating Disorder consultants and the NHS waiting list was horrendous. So she saw no-one in the second year of Uni and I worried like hell.
In the third year she had a work placement in Sheffield where we saw a private consultant again - and he was both unpleasant and useless. There were young girls living temporarily in this unit who were clearly very, very ill and I despaired. My wonderful DD was messing up her entire life - and for what? She had no explanation as to why she was doing this. She went back to Uni for her final year and at exam time she begged me to go and stay with her for the exam period and between us she managed to achieve a very high 2:1 degree - but it cost me £2k for the hotel I stayed in!!!
She got a graduate traineeship with a major bank and moved to Sheffield again. She referred herself to the NHS and good fortune shone at last. Her new GP was supportive. A new unit at the hospital had just opened with a young, enthusiastic head, and she got a referral there. To this day neither my DD nor I have a clue what "magic" he performed but she said that he listened to her, seemed to understand rather than ridicule, prescribed and monitored some medication (that altered the dopamine levels in her brain) and bit by tiny bit she improved until one day, about two years later and then aged nearly 24, a light came on in her brain.
Today she is nearly 46, married to a wonderful man, has a good career and two fantastic teenagers to whom she is a marvellous mother. She's still a tad on the skinny side and does have some physical health issues that I wonder if they originated from the damage she was doing to her body back then - but I have no proof of it.
I did find BEAT useful, it was good to get hints and tips when things went wrong and have some support - and give support - from others going through similar.
I hope your DD gets the help she deserves - and be kind to yourself too. I blamed myself at the time, but DD insists that I did nothing wrong.