Take a look at these steps and use them to help you get through the trauma. This can be an exceptional painful issue and the emotions are very intense so use this as a checklist to help yourself get through the event.
First and foremost, take a deep breath and some time. Do not let yourself have a knee-jerk response. Think! This is especially important in long-term relationships. Sudden reactions without thought can lead to consequences you might regret.
Give yourself some mental space before you take any action. Talk to someone. You are not alone. Statistics are sketchy and vary widely, but many surveys have been done on cheating and they indicate that between a fourth and half of all married people will or have cheated at one time or another. Do not blame yourself.
It's easy for people to start looking at themselves for reasons why their partner cheated... nothing good will come of that. Issues that lead to cheating sometimes involve both people, but that's certainly not always the case.
However, it would, help, at a later date look inwards too to find out why your partner looked elsewhere for comfort. There could be certain grey areas in your behavior which could have led to such actions. You have to remember that most humans like a monogamous lifestyle, as it brings about so much of happiness & security. However, there are a few who would not conform to this.
Determine whether you were actually cheated on. Ask yourself these questions: Were you officially boyfriend and girlfriend at the time this "cheating" occurred? Were you officially monogamous? If not, you cannot be sure that your significant other knew what he or she was doing would offend you, in which case you might want to consider less confrontational options. Talk to your partner. Let your concerns and fears be known. It might come out that nothing at all happened, or perhaps something did happen and coercion was involved (workplace sexual harassment, for example, which needs to be discussed openly and immediately to ward off future occurrences). There could be a substance abuse or psychological issue that needs to be addressed (sex addiction is very real).
If help is warranted, you might want to support your partner in getting help - that could prove therapeutic for both of you. However, substance abuse is not a valid "excuse" for inappropriate behavior and you absolutely must not permit the "yeah but I was drunk so it doesn't matter" argument - stand very firm on that. Ask yourself if you will ever be able to look at your partner the same way. Infidelity doesn't mean much for some, and some people have more than one physical relationship and it doesn't suggest a shortcoming in their relationship with their steady partner, but this is rare. Infidelity often indicates boredom and dissatisfaction with the present relationships.
Dealing with a partner who doesn't want you in the first place, or one who doesn't mind hurting you is ridiculous. Dump him/her if this is the case.