Christians must forgive - for our own personal good, primarily.
Unforgiveness is resentment - close-cousin to spite: to desire harm against another person or Being (harm even at the cost of our own suffering). And spite is probably the worst, most insatiable of sins.
Failure to forgive will keep us out of Heaven, because we will not even want to dwell in Heaven - which is a place of love and without spite. Someone who prefers to sustain his own resentment - even on a single topic; and who will not accepts the cleansing grace of Jesus Christ to be purified of it at resurrection... Well, such a person has damned himself.
(Unforgiveness is not-to want-to forgive - forgiveness is the opposite.)
Forgetting is not the same a forgiving - although forgiving (when effectual) puts an end to that grinding resentment which gnaws at the soul. When one has forgiven there is no particular reason to ruminate obsessively - therefore one may think on it less, or with less extremity...
On the other hand, forgetting may be undesirable - we may need to learn.
For instance; it may be right and desirable to remember when someone (or some institution) has lied to us. We ought to know liars, because (unrepentant) liars are servants of the devil; and therefore we may need to regard them differently.
We need, for example, to recognize when someone (or some institution) is using language in order to manipulate us, and not to communicate truth. This means we are dealing with evil; and therefore it is desirable for us to remember that this is a liar.
We must forgive the fact they have lied to us - because not to forgive means deliberately nurturing resentment - which will act on us as soul-poison. However, we should not forget the fact; because that They are 'a liar' is spiritually relevant and important.
But what if "I can't forgive"?
Well, that is never true - for forgiveness is the inner act of wanting to forgive - forgiveness just-is the recognition that we ought to forgive, and the willingness that we should be enabled to forgive.
While we are on earth everything - including the human mind and body - is subject to change and mortality, and the limitations of our physical situation (the constraints of our body and mind).
Therefore, the actual 'process' of forgiveness, which is eternal, comes only after our death - at the time of resurrection.
Until then, what matters is our recognition and willingness.
Recognize that you must forgive and be willing to forgive - but do not try to forget matters of spiritual importance.