I'm not sure I see the problem here? You have feelings for her, she's single, you both apparently enjoy spending time with each other, which is a solid foundation for a relationship. What in the world is stopping you from asking her out?
edit: Actually, let's examine this even more: You've already asked her out, but were turned down due to the fact that she was in a relationship. So she obviously already knows that you have feelings for her, yet she is having no issues whatsoever in spending time with you now that she is single. That means there is no awkwardness there, which normally there would be, which to me is a bit of a sign that she might like you as well.