What does this woman's husband think of your relationship with her? Is he amenable to it? Does he know about it? Are you friendly with him?
I won't assume that all you aren't being open about things, but because you haven't specifically stated that, I have a feeling that you aren't. And if that is the case, this is a bad situation.
So - scenario 1: all is open and everyone knows what's going on. Ask your sweetie what she means by "defensiveness" - get concrete examples. Agree on specific signs of progress and goals that you will work towards achieving. If she's doing something that triggers your defenses, work through that with her and ask for reassurance if you need it. It may be helpful to ask for input from her husband.
Leave off any discussion of moving for now. That's a major decision, and I really think there are several steps you should take before considering it.
Scenario 2: this is an extramarital affair. END IT. Nothing good has ever come of any such relationship. You are completely in the wrong and contributing to the destruction of a marriage. It doesn't matter how many affairs the woman (or her husband) might have had before, or if she would have had other affairs if you were not involved with her. YOU are acting dishonorably. And if she were to end the marriage to be with you, how could you ever trust her? How could she trust you? Both of you have proven yourselves to be completely immoral! Trust is the basis of intimacy. Without intimacy, you'd just be flatmates with benefits.