As long as you're stronger than her I don't think using physical power is OK. You can slap her two times harder than she did, can't you? Also you were the one who cheated on her. You made the mistake, not her. You should find a way to slow down your nerves.
Well that was definitely a knee-jerk reaction... Both of you did a wrong thing. The difference is that you realised that you shouldn't have cheated on her but she keeps insisting that her slapping you was okay. Her reaction was violent but you cheating on her is kind of worse, so be happy that she forgave you.
Both of you should work on your relationship and be willing to change, things aren't working as good as they should've.