What's done is done.. your not going to change it now mate.. If I were you, I would leave her because for a guy to do that to his girl is unacceptable and is not "man enough" to take care of her.

No matter how much you tell her, there is NO WAY you are going to take that back. So I recommend telling her what happened to the extent that it doesn't hurt her and that you ensure that you won't hurt her again.

In saying that, I don't think you really deserve to have anything to do with her UNLESS both of you have some sort of love-connection which I doubt and for this reason you should move on thinking about this and ensuring it doesn't ruin your life or another girl's life