When you get caught cheating, the whole world comes crashing down. Your girlfriend's head will seemingly become a whirlwind of pain and anger, and there's nothing that you can do but stand there and tell her you're sorry. No amount of apologizing can change her mind, and nothing you can say will ease her pain. In other words, a mere apology just will not suffice.
While there is no one reason why men cheat, most men that engage in these practices are either looking for something that their girlfriend is not providing, they actually desire to be with different women or they are just commitment phobic. All men know or should know that cheating is wrong; however, in troubled relationships, cheating can be an easy alternative to the burden of a relationship breakup. Make no mistake about it--when your girlfriend finds out you've been with another woman, she's going to be furious for a while. Some women will demand to know what happened, how long it's been going on, and what you've been doing with this other woman, etc... As your girlfriend rages against you and the emotions fly, you'll begin looking for ways to undo what you did. The bad news is that you can't.
One of the worst things you can do after cheating on a girl is try to blame something or someone else. In a situation like this, she's concerned with her own emotions, and the last thing she wants to hear about is how "badly" you feel. So avoid telling her stuff like "I feel terrible that this happened". If you truly love and care about your girlfriend, you should feel some pain and guilt. When she is sufficiently calm, make a heartfelt, genuine apology for cheating on her. Take responsibility for cheating on her and don't defer the blame onto your lover, how much you had to drink, or whatever. When you agree with her that this is definitely your own fault, her anger can finally start draining away; however, don't think that just a couple of days will be enough for her to get over what you did.
Some women will cheat on you in retaliation ( with a guy who's hotter than you) or just choose not to forgive you and move on. Even if you're lucky enough to have a a girlfriend who is truly forgiving, loving and understanding --the relationship will change forever. Make no mistake about it: cheating automatically kills the trust that your lady had for you. Her allegiance and dedication to you will drop immediately, and even if she does not state it verbally, you'll see the change in her demeanor when she's around you. She will always have trust issues with you from that moment, and although she may try to forget what you did, she will always be hurt.
Keep in mind that although she may forgive and forget, your girlfriend's family and friends will always have a longer memory than she does and will still hate you long after she forgave you. You also have to deal with the possibility of the fling that you hooked up with wanting to be your main squeeze and possibly causing even more problems for your already damaged relationship.