To cope with infidelity, both partners need to make a decision about whether to stay in the marriage. It may seem strange that the cheater usually does not want out of the marriage. Thus, the spouse victimized by the behavior is plagued with an overwhelming decision. He or she does not have to make a decision right away; rather, it is advised that the person take time to consider whether they can forgive their partner and work to continue the marriage. If they choose to stay, they will need to undergo marriage counseling, and work on communication and rehabilitation of trust.
Grace3, I work with couples all of the time that are looking to overcome infidelity, trust issues, and even worse issues. What exactly are the circumstances of the infidelity, and what are you doing to mend the situation right now? If you would be more comfortable talking with me directly, I can give you my contact info or you can find it in my article "When is it time to call it quits?"
Seek professional counseling and seek reconciliation - reprioritize your marriage. Take lots of time to talk through your issues - how your marriage became broken, the downward spiral that led one of you to cheat, and work towards forgiving one another genuinely. This can be very difficult and may be a very long road, but marriages can be saved.