The Lord will forgive as He judges appropriate, but we are commanded to forgive everyone. This can be very difficult at times, particularly when consequences of the offense are long-lasting. Simply trying to convince ourselves that we need to forgive is often not enough to change our hearts.
To help us change our hearts, we can pray for those who've offended us. We can find ways to be a blessing to them. Rather than passively reminding ourselves to forgive, we can actively show our love to them. Every sin we commit directly affects Christ, yet He responds by "making His sun to rise on the evil and on the good."