ask her out- if she agrees then yay.

if not- it's time to move on as she's using you for emotional support and the moment she no longer needs you, you guys will drift apart. she's probably not a horrible person that will throw you aside but the friendship is based on your need for her love and her need for a close friend to support her. If she can find someone to be close with that she doesn't have to worry about accidentally leading on, she'll end up closer to that person. and from then on- slowly but surely, she'll just be more and more distant without even noticing what she's doing.