I have had this exact same situation, twice in fact. Lots of “I can’t” and “I don’t know” and general negativity throughout the lesson!
The first case was with a highly anxious 14 year old. I tried everything… for 2 years.
By everything, I mean I was gentle, patient, went at her own pace… then I would try to push her a little… she sometimes responded well but other times the opposite. I tried the tough love… the counselling method… just chatting about her day. But nothing really worked in the end unfortunately. She had a beautiful voice but I think her illness got worse so she started to regress. This is probably an extreme example and there’s only so much a teacher can do until out of their depth where illness is concerned. My best advice is to keep in contact with the parents and make sure the student is always comfortable.
However, yours is 18 and therefore an adult. I also currently teach a 21 year old with a personality disorder. Lots of negativity and again, a stunning voice but keeps shying away from each challenge I give her! I try to just let her do what she’s comfortable with in order to build confidence, before jumping in at the deep end and getting her out of her comfort zone (like I do with most students!)
I would have a chat to her and find out her likes and dislikes and see how her life is in general?! Also get her to talk about what makes her feel passionate about music. Aware of course, that you can’t pry too much when it comes to the “life” stuff, but just be open about listening, saying she can confide in you and that she won’t be judged. This can lift a huge weight off a students shoulders (which can take time!) and she’ll learn to trust you, which should result in her being less afraid and more relaxed and able to set her voice free!
I am still in this up and down process so any further advice for my 21 year old would be great!