The relationship will not move on without an apology. If she hasn't given you one, ask her. It might be difficult to tell if the apology is heartfelt, but ask your girlfriend for some time to process all that has transpired. When the time is right, your acceptance will begin to mend the relationship. Keep in mind that this might not be the end of the issue. Even if she is truly sorry, your relationship will likely need work, which will require both of you to work together. If you decide to work through the incident and keep the relationship going, talk about trust. The hurt that occurred will drive a huge wedge between you, but deciding to be together means regaining that closeness. Communicate with one another and express changes that should occur in the relationship. Avoid stripping each other of any and all privacy (i.e. checking cell phones, demanding social media passwords); the relationship must have the enjoyable will of being with each other. Being betrayed by someone you care about can leave you seething with anger. It's likely that the memory of this incident will remain with you for a long time. That feeling of anger must be replaced by forgiveness, which in this case forgiveness means the release of anger in hopes of happiness. Mending your relationship will take time, and you'll feel more patient as you forgive her. A new relationship must be built. It's up to you and her to move past the incident. Stay true to your decision. Help each other by looking forward to a new and better relationship. Do not dwell on the past. If your relationship is going to work, do not take any fight as an opportunity to bring up the mistake she made. Your decision to be with her should be driven by better communication and future-oriented thinking. Discussing issues with her might be difficult. Even though you you are moving towards a better future, an incident of cheating can be too big for you to handle on your own. Seek a professional therapist or relationship counselor if you feel as though you can't get something off your mind. Don't force your girlfriend to go to counseling as well, but invite her with the best interest for your relationship.

Summary:
Accept an apology. Express trust issues. Understand forgiveness. Discuss your future together. Move forward. Seek counseling if needed.