Hi Monkey,
Try calmly talking to her about your feelings. Don't accuse her of doing anything wrong, just let her know that making out in public and being slapped is not something that works well for you. It's not something you're comfortable with. Ask her how she feels about it.
My first girlfriend liked to slap a lot too. One time I made a joke and I was slapped hard in my face out of nowhere. Soon after that I broke it off. I really don't like being smacked, especially not in my wonderful face.
Later on I found out she was having a lot of issues at home. She admitted that slapping me was probably her (ineffective) way of dealing with it.
There are plenty of girls out there. You're young. If you feel a strong need to break up, go ahead. There will be others.
