You are together now so I would make it clear that you are not one to give up because you have the love of God in your heart and be an example to her. Tell her that if she feels the need to leave you can not get back with her again and you have no plans to leave her. You are in for the long term relationship if she wants less then she needs to look for someone else. I think that maybe she was scared of a long term relationship, didn't know how to deal with someone that really cared, etc... but if and when she leaves again then you know it's a pattern she can not break. As the others said I think counseling and partner counseling would be beneficial.