Forgiveness is hard for us as Christians, but not as hard as many may think. We've all had experiences where we feel like we've forgiven the person, but then continue to get mad at them over and over again. While sometimes it may be the case that our forgiveness the first time wasn't complete or that we may be fighting the tendency to continually drag up the past, I have a feeling that more often than not, the case may be something totally different.
It happens when one party refuses to talk about the situation and resolve it. So many people think that if you ignore a problem long enough or try to forget that it happened, that it is a substitute for working through an issue and forgiving the other person. That is a lie fed to us directly from Satan! Time doesn't heal wounds...time just stuffs them down and allows them to fester. And forgiveness is not forgetting. It is actually canceling a debt and setting things right again.
If you are having trouble laying an issue to rest and truly feeling like forgiveness has happened, ask yourself, did you actually resolve the issue? Are you showing your emotional or spiritual immaturity by not facing the issue with the other person and putting it to rest? Is the other person showing emotional or spiritual immaturity by refusing to work the issue out and talk about it? In fact, if the latter is the case and you feel like you have to forgive the person over and over, perhaps you have forgiven them for the original offense, but not their immaturity they are displaying over and over again by avoiding the issue and refusing to deal with it.
Unfortunately, some people really are too weak or prideful to sit down and talk about something that has occurred. Some people don't have a strong enough relationship with Christ to actually put in the work to set a relationship right or to confront the issues between two people. All you can do is keep forgiving them for their failings. When Jesus told Peter to forgive people unendingly, I suspect this is the primary type of forgiving that He meant, the constant forgiving that has to happen when one person refuses to face their sin, weakness or weakness of character.