In any relationship, once trust has been broken, it can be extremely challenging to get it back. Forgiving a lying boyfriend is more than just a matter of accepting his promise that he won't repeat the action. It takes time to allow yourself to really forgive him and he must be willing to accept certain conditions in order to move forward in your relationship. It is only after these conditions are consistently met that both of you can begin to heal the relationship.
Talk about the reason for the lie. Guys lie for numerous reasons, including the desire to avoid an argument about the infraction to feeling ashamed about what they've done. You must discover why he lied in order to understand whether a lack of communication is to blame. Once you've discussed the reason for the lie, you may find that he felt as though you were unapproachable, a problem that can be resolved by developing clearer, more open communication.
Be expressive, but not mean-spirited. It is important to communicate how you feel about the fact that he lied to you. However, you must communicate those feeling effectively, in order to make a lasting point. Use strong feeling words and stick only to how you feel about this particular action. Refrain from being mean and don't bring up past infractions. Stay in the present.
Make a plan to move forward, with the understanding that his ability to be consistently honest with you will determine the future of your relationship. This may include honesty about the small things, such as checking in at certain times of the day or making his whereabouts known. It may also be more in-depth, such as allowing you to keep a tighter reign on joint financial accounts. Whatever the plan is, communicate the terms clearly, outline the consequences if he lies again and obtain his commitment to follow through on his promises.
Seek counseling for seemingly insurmountable issues. If your boyfriend lied about something small, like drinking one too many beers with the guys or forgetting to get the oil changed, that is something you can work through. However, if the lie is regarding a situation of infidelity, you may need to seek professional counseling to learn how to forgive problems that undermine your future together.