It can be extremely difficult to rebuild a relationship after you have lied. Couples break up and get back together all the time, but if the breakup was caused by a betrayal or dishonesty, reuniting will take time and effort. Don't expect your ex to forgive and forget right away -- in some cases it takes months to be able to let go of those hurt feelings and start afresh.
Apologize for lying. Whether you told what you thought was an innocent white lie, such as telling your girlfriend you were working late when you were actually meeting your friends in a bar, or committed a far more serious betrayal, accept that you were in the wrong. Keep your apology simple and don't try to excuse your behavior. Tell your girlfriend that you made a huge mistake and that you are sorry. It is only possible to rebuild trust when you admit your mistake and take full responsibility for it.
Let your girlfriend get her feelings off her chest. Listen to her without interrupting or trying to defend your behavior. You can't take back the lie, but you can give her the chance to explain how your actions made her feel. Accept her point of view without arguing, and tell her you understand why she feels hurt.
Ask your girlfriend how she feels about giving the relationship another go. Give her the space and time she needs to make her decision. Don't get angry with her if she is unable to give you an answer right away. Accept that it could take a while for her to make her mind up, because she may not be sure whether she is able to trust you again. Rushing into things before she has completely forgiven you will not do either of you any favors. Any residual anger, distrust or resentment could cause your relationship to deteriorate even further. Rebuilding your relationship must be from a starting point of forgiveness and trust.
Be honest. Not just with your girlfriend, but with yourself and everybody else in your life. Let your girlfriend see you behaving like someone who can be trusted, and she will be more likely to believe that you won't betray her again, says Dr. Mark Goulston in "The 6 Secrets of a Lasting Relationship."