Right, me and my girlfriend have been together 10 months now, and on New Years Eve, she went out to a party, and cheated on me with a girl. Not just kissing. Turns out 7 guys filmed it, and her excuse is she wanted to feel like a porn star.

How is this meant to make me feel? I'm at a lost end, and really don't know what to do with myself. Forgive and forget?

There is absolutely NO excuse for this behavior; I would say you two have
A major issue now.

Tell her if she wants to feel like one then you treat her like one

Dump her! And let her feel that her actions have consequences!

Of course forgive, and forget. AFTER you dump this want to be porn star.