There are many reasons that people stay in relationships that they shouldn’t. But you don’t indicate that this woman is being abused or threatened or manipulated into staying. Instead, she seems to be juggling the two relationships and even lying about it. That suggests that she likes things just as they are, with two men interested in her. She isn’t making a decision because two relationships is more satisfying to her than one. I can’t offer any explanation for that without knowing her.
I can suggest that since you’ve only been involved for 6 months (and involved in a way that has been painful to you), you would do better to take a big step back. Ask yourself why you are willing to accept conditional love and “waiting” while she makes up her mind. You deserve so much better than that.
Take her at her word. Pull back from this relationship and make yourself available to find someone who can love you wholeheartedly and without these mind games.
I wish you well.