If you discover your girlfriend is cheating on you, you will naturally have some things to deal with in your mind, namely anger, low self-esteem, depression and feelings of betrayal. Before you forgive her and begin any attempts at saving the relationship, you need to know the ways to distinguish between an unfaithful woman who will do it again and a woman who is truly sorry.
First, you need to examine what was wrong with your relationship in the first place. There are many reasons why women cheat. Determine her reason for cheating and consider whether you have anything to work on in the relationship that may have led to her cheating, like possibly lowering her interest level. If you love her and you feel that she truly does love you then both of you can work through it, even with the use of couples counseling, if necessary.
There is a big difference between a one time mistake and a long term affair. Make sure you know everything. When you ask her questions about it look her straight in the eyes and see if she is looking back into yours. If she isn't then there may be more she is not telling you.
If a significant amount of time has past since the incident then weigh what has been built since with what you would lose if you went your separate ways. Consider your physical health and emotional well-being. Could she be putting you at risk for sexually transmitted diseases? Can you deal with being one of many possible sexual partners that may be infected with something? Regardless if you can or not, consider getting an AIDS or other STD test.
You must also determine whether or not she came back to patch things up with you because she truly loves you or because things didn’t work out with the other guy she cheated with. If you decide to forgive her and take her back, make a list of "rules" never to be broken, go over them with your partner and tell her that she must do these things to remain in the relationship and regain your trust. If she agrees then this list will help in the following months.
As time passes images of the incident or thoughts of "is she cheating" could pop-up in your head. Consider whether you can actually continue in the relationship regardless of whether she will do it again or not and if so, separate yourselves from any bad influences or images that will remind you of the cheating and the bond of trust that was broken.
Remember that you are not the only one going through this, in fact you will come to find that many of your friends or family have been hurt by cheating. Take advice and listen carefully to your lady, but ultimately you have to do what you feel is best for yourself. Remember--it's how hard she is willing to work to regain your trust that really counts.