Dealing with an unfaithful partner can be difficult, especially if your partner was physical with someone else. While some people choose to end relationships where cheating was a factor, sometimes relationships can be salvaged if the cheater is genuinely interested in changing and if the partner is ready to forgive. To have a successful relationship after infidelity, you must forgive your girlfriend for the past and move on.
Ask your girlfriend if she's willing to change and if she wants to save the relationship. If she does and you feel that the relationship is salvageable, then you can start the process of trying to heal and forgive.
Talk about the issue. The first step to resolving conflict is to communicate. You may need to talk about this a lot in order to get it off your chest and to move on. Your girlfriend should allow you to discuss, ask questions and talk about it as much as you need to so that you don't bottle up feelings and resent the situation. Understanding her reasoning and having her share details may be part of the healing process.
Realize that nothing will change the past. You have to accept that it happened and know that although you can forgive, you'll likely never forget. It can be tough to realize that something this painful will never completely go away, but time can heal and you can learn to trust again if your girlfriend is willing to prove herself. You must learn to live with the past, but move on to a brighter future.
Ask your girlfriend to to be honest with you. If you feel like she's being shady or withholding information about where she is or who she's hanging out with, let her know that it makes you uncomfortable. While you shouldn't be controlling or ask her to disclose her every move, a simple text or call from her that says, "Hi, honey! I'm having dinner with a coworker tonight," would probably ease your mind and leave you more likely to move forward after the infidelity.
Be patient. It may take time for you to move past your anger and resentment towards your girlfriend. Time heals, especially if your girlfriend is committed to showing you that it was a mistake and she's truly sorry.
Don't dig up the past. After you've healed emotionally, you may find this to be an easy topic to bring up during arguments. Know that this isn't a fair way to have a productive disagreement and that by bringing up such a touchy subject, you'll not only continue to resent the past, but you'll also hurt her in the process. If you truly want to forgive and move on, let it go.