If she is using this guy to make you jealous, so that you will propose marriage, then she is a dishonest person, and not someone you should marry. More likely, she is using the "it would be different if we were married" excuse because she knows how you feel about marriage and therefore she has a convenient way to shut you up.
You are right about the no-win situation. If you pursue, she will pull away. If you ignore her, she has more time with the other guy. The only way you win, is for you to control how much time she spends with you. Meaning zero.
Do not call her. Do not make time for her. Make yourself very very busy. This is called "going dark." It will either make her miss you, or it will show her that she doesn't have to try to spare your feelings.
