I probably don't have grounds to give advice in this situation, as I've never been in yours or your student's position before, but with you as the teacher and her the student it may not be a good idea to pursue any kind of physical relationship. It can be good as a young person to have an adult to confide in and talk with who you feel close to and who's thoughts on things you respect. However even with your student being of legal age it still feels a bit like seeking an actual relationship could only complicate things in the student's mind. A teacher or any older adult figure who's in a position of superiority over a young person can become a powerful mentor to them. And while we may experience crushes on some of our mentors, or even fantasize about them, it doesn't mean that making it a reality could be healthy. It's not a question of age difference really (though there is a maturity level that can come into play with someone fresh out of high school) but there is something to consider about how your student has viewed you growing up in your classroom, learning things from you, trusting you. That's a great responsibility and one that shouldn't be taken lightly when considering moving forward with something that may only be a fleeting sexual desire. I'm not trying to say what you and her have is a fleeting thing, only that you should take any decision you make about this very seriously, not so much just because of your job (though that could be a factor) but more so for the sake of your student. She may be 18, so legally an adult, but as far as thinking fully as an adult and making these kinds of adult decisions about someone who's been a kind of therapist in her life could prove harmful to her if/when the relationship finally ends. I know the person I was at 18 was not who I would consider a fully matured adult. She may look up to you a great deal, and take anything you think (both positive and negative) to heart very deeply. I can only imagine if a mentor or teacher I cared about and thought highly of took an interest in me sexually. I'm sure I'd be very excited, but with that excitement would be a feeling of nervousness, wanting to please them, what if I do something wrong, what if we fight, what if they stop loving me, all those feeling become amplified when you're talking about someone you've looked up to for so long. And what if her thoughts and feelings on being in a relationship change as she grows up? She may just have a crush on you right now, because you're a mature, caring, successful adult who's taken an interest in her feelings and her life.

As I said I obviously don't have any experience with this kind of thing, and I don't know your interactions together are, but if I were a teacher, or any figure who was in a kind of superior role over another person (be it boss, psychiatrist, therapist, professor, doctor, lawyer, etc.) I would think long and hard about dissolving that relationship barrier with a student, patient, client, subordinate. It's not the kind of thing you can take back when it's done and it's something that will stay with them, and possibly shape their future relationships for years and years to come.

In the end I would say just stay friendly, be supportive, give advice and an ear to listen and shoulder to cry on, but maybe back off on the whole physical relationship thing, she may need your guidance, your friendship and mature knowledge of life helping her through this new and difficult chapter of her life more than she needs a full fledged intimate relationship. Anyway, that's my thoughts on it, I wish you both the very best though, it truly doesn't sound like an easy thing to think about by any means.