GF found old text of when I 'cheated' when we just got in a relationship

The texts were when I was just 1 month into the relationship. That is 6 months ago at the moment.

I forgot to log off facebook and she read the conversation with a girl from vacation she always had a feeling was trouble. The girl texted things like 'I enjoyed kissing even though you don't want to admit it'. She kissed me but I didn't kiss her back. Also stuff about hooking up if my relationship would end. Crying ensues.

How do I handle this situation?