Your girlfriend needs to be told that she is no longer allowed to put her hands on you during a fight. She may not slap, kick or hit you any more. If you put your foot down with her and she knows that you mean business, then your word is the law and she will know that you mean business.
If you don't get this acting out under control before the wedding, it will only get worse after you tie the knot. You both need to learn how to stop acting on your impulses and learn to speak constructively when you're angry. My book, Till Death Do Us Part (Unless I Kill You First): A Step-by-Step Guide for Resolving Marital Conflict explains how to handle angry feelings constructively.
I would also suggest that you ask your girlfriend to tell you what you are doing to infuriate her so much. Even though you are doing something to piss her off, that still doesn't mean that her behavior is proper, and, yes, she needs to learn how to tell you what you've done to upset her using her words, not her hands.
At the same time, you need to find out what you're doing to push her buttons and then stop pushing those buttons. In a calm moment, ask her to clearly and calmly describe to you what you are doing to anger her and what she would prefer that you do instead. When a woman gets as angry as she is, I can assure you that you aren't doing a good enough job listening, understanding, and accepting responsibility for your own actions.
So, she needs to learn how to properly tell you what you're doing wrong and you need to accept responsibility for what you're doing that 's pissing her off and then be more responsive to her in the future. I suggest you both use my online consultations, to help you pinpoint why you're fighting so much.
First find out if you're fighting because you're incompatible, using my Are We Compatible consult; then, find out if she has childhood baggage that 's fueling your fights, using my Why is My Relationship with Her on the Rocks?' consult; and last but not least, you need to do my Psychological Check-Up (coming soon) to find out what you're doing that is rocking the boat and to receive suggestions on alternate ways of dealing with her.
I don't think you are wise to marry each other until you get past this very serious problem.