In my book Unpacking Forgiveness (Crossway, 2008), I emphasized the conditional nature of forgiveness. Christians should always be willing to forgive. We must love our enemies and show them love. But forgiveness can take place fully when the offending party repents.
Conditional forgiveness is a topic addressed by many others. Here is a survey of excerpts of what others have said about conditional forgiveness. Of course, not all agree perfectly with one another. Those motivated to investigate further, should read the entire context of the quotes.