You know, once the trust is gone, it's gone. I am sorry to have to tell you that, but I think you already knew.
Now the tricky part......Everyone we love will let us down one day. It may be a little hurt or it may be a big hurt. Regardless, someone we love will hurt us. To move beyond that hurt takes a long time and a forgiving heart.
If you really want to stay with your guy, I would suggest talking to someone (a professional) about how you feel. I bet you can move beyond it and just maybe.....The two of you will be a stronger couple for having gone through it.