This is a sad, but not uncommon, story. I do sympathise with you and your husband.
Of all the problems that are difficult to treat, drug taking is one of the worst. I know that as a retired Border Force officer. Stopping taking drugs is so hard. Almost impossible without professional help, and well nigh impossible if the drug-taker doesn’t want to stop. Your daughter clearly wants to continue taking them, so nothing you say or do will change that. Until, and if, she has a change of heart, she will continue in this vein.
There is nothing you can do to change her. Even if she swears she is clean, you probably shouldn’t believe her. People seldom come off drugs without massive help from a professional, and even then, they need to want to.
So, what can you do? Apart from being there for her, not a lot. Her doctor who prescribes the drugs will not discuss her with you. That said, I would be inclined to write him a letter laying out your concerns and asking if he can do anything at all to help. He might be able to persuade her to speak to a therapist where she can talk about her hang ups, why she needs drugs etc. The therapist might have a way to help, but if your daughter doesn’t want to be helped, that’s another story.
All you can do is support her as best you can, be there for her, don’t criticise her or try to make her change. You are doing the right thing not deserting her. She is lucky to have good parents and believe me, there is nothing you have done to cause this so do not blame yourselves. Over the years I have dealt with many youngsters (mainly females) who took drugs. It always amazed me how many came from affluent homes and/or close-knit ones. It is very seldom the parents’ fault.
Don’t give up on her, or yourselves. Keep the door open, keep up contact as best you can (no mention of drugs if you do speak to her). She’ll break contact if you lecture her. I used to try to have gentle conversations with the youngsters I came across when arresting them at work, and most said they left home because parents tried to ‘cure’ them (not possible), or change them, also not possible, or made their lives a misery instead of trying to understand.
Hang in there! It’s hard but you’ll get through this.
My thoughts are with you..