Most Helpful Girl
while i don't condone violence as a reasonable response for either gender, you cheated and you need to deal with the consequences.
(i'm not saying that she was right to hit you. i'm saying that your cheating on her, right in front of her no less, was slimy and wrong and that you hitting her back effectively killed any chance you had of her forgiving you.)
my advice to you is to leave the poor girl alone. she deserves better than you, and you need more help than anyone on the internet can give you.