If you love her, love her as she is without trying to change her.
If you feel the need for her to change something, then you don't love her, but your own idealistic vision of her.
If the smoking is something that you can't deal with, then you should seriously evaluate your relationship.
You can't change her, plain and simple. Only she can change herself, whether it's quitting smoking or getting more organized or going to college. It's entirely up to her. If she chooses to quit smoking, support her and help her in any way you can, of course.
But ultimately, it's up to her to make that decision.