I know how bad you must feel, someone you like asks you out then changes their mind. What's wrong [sorry if this hurts] isn't you, she's just gotten out of a relationship. She needs time to heal. Many people jump into relationships to fill a void within themselves. The thing is they need to be by themself for a while to come to terms with things. All you can [and should] do is be their for her. When she's ready, she will be happy for what you did, and ask you out again.
Now, if you choose the magickal route [which you shouldn't do, but everyone's morality is different so just think before you do] I would go with a honey jar or possibly a spell to help her heal since she apparently likes you, she's just hurt right now.