Yeah, forgive her, take her back. Then she'll know that in the future if she does it again, you can both just have a good cry about it, then move on with your (exclusive?) relationship. No harm, no foul.

Open your eyes man, she was shopping around for something better at the beginning stages of the relationship- that in and of itself is not a huge deal if you guys weren't so serious then (women shop), but think about it. For the duration of your relationship, while you were supposedly building TRUST, she kept this from you the entire year.