Thank you for your question. Without talking to her, and discussing this issue with her, there is absolutely no way we can help her.
Committment issues are most often deep seated issues related to primary relationships growing up, especially with the father. (can be mother or father) It can also be related to how she was treated growing up.
This is not a quick fix.
One method is that a person can just slow down and take it easy. If her committement issues are simply fear of he upcoming marriage, then go slower. Build trust, get her feeling comfortable with you. If this does not work, then her "committment issues" may be deep seated as I mentioned earlier.