If she isn't the one, then she isn't the one. This may change in the future but right now it doesn't sound like you two are right for each other.
What's the source of your guilt? The fact that you feel this way for another woman already indicates that you don't love your ex the same way anymore. If the relationship is in this state it's better to break it off sooner so you can both start the healing process. In your case, hopefully with the help of a certain someone.
