There's always a chance but it doesn't mean you should be hanging around waiting for her. Youth does play into this to a degree, and nothing can change that but time and experience. If my 32 year old self could have a conversation with my 23 year old self, I think there would be a whole lot of finger-wagging and raised voices going on...boy, was I head strong. But anyway, let her go play with this new-found "love" of hers. Don't interfere, don't let it weigh on your mind and certainly keep her out of your bed.
You have to make the statement, whether spoken or not, that you will not be a yo-yo on a string for her. Normally, when a girl says she wants to spend the rest of her life with a gentleman, she means it. If her heart is set on it, her mind will eventually catch up.
She could be going back and forth because both of you are making demands in some form and she doesn't want to feel controlled by either of you. If she knows you're there to run to when things get rough with him, she'll eventually start sabotaging both of the relationships to create a safety-net that suits her needs when she needs them.
: Don't be that guy. Tell her no. Don't let her work you and when she figures out what she wants, you'll still have a clear head about you and can make a wise decision instead of one made in the heat of the impending drama this girl is about to create. How do I know this? I used to be her...and then I grew up.