Hiii I would love to add my opinion as someone around your stepdaughters age (a little older, not by much though)
Granted I know nothing about her so this is all speculation based on my experiences
First, being afraid of driving does not equal lack of independence. Driving is scary, other drivers are scary, and vehicle accidents are a top cause of injury and death. If she has attempted to drive and failed, chances are she had a bad experience, or the people/person she is being taught from may not have a teaching style that is the best for her. I do not drive either, and it's definitely not for lack of trying.
A lot of where I personally lack independence is directly related to my anxiety. I have generalized anxiety and I have even struggled with agoraphobia because of it. Her issues with independence may lay under mental health issues as well, which are not always easy to see or help.
My advice would definitely just be supportive. You sound like you're already on the right path with that. If there are things she seems uneasy doing, try doing them with her first, and then get her to do them on her own.
I know for me if I've done something once with support, it makes it so much easier to do it by myself. Keep encouraging her and being there for her. Also, as a step parent please know that most of the responsibility should fall on her biological (or adoptive) parents/guardians. Make sure you and her are both getting the support from them as well.
One thing that REALLY helped me is movingly away from my parents. I moved three hours away (and then even further) and it was honestly the best thing for my independence. But the only person who can force her in that way is herself.