At first glance this quotation may seem ridiculous. Clearly we cannot heal a broken foot by deciding the foot is fine. What meaning or use can we gather from this then? A logical first step would be to try and understand what exactly injuries are. Despite "injury" being a common word it is difficult to define. To get a grasp on it lets take the example of someone falling and bruising their leg.
This person starts off in what could be called the initial state. Then an event occurs (the fall) and they end up in an inferior state (bruised). The tricky part of this understanding of "injury," is that there is no clear way to determine what is an inferior state. All of our judgments on this seem based off imagining that the "injurious" event didn't happen, examing what the object would be like then, and deciding whether we would prefer that the event didn't happen. If we'd prefer that the event didn't happen then we call the event injurious. To this extent, injuries and our perceptions of them are the same.
We can now understand how rejecting an injury can remove it. To consider yourself injured you have to compare yourself to an imaginary version of yourself, and decide that you come up lacking. This is already sounding a little ridiculous but let us continues. To what end would we make this comparison? We cannot change the past so we must look to how being injured can change the future.
The first and simplest way is that we can learn how to act from being injured. If we examine the causes of our injury we can change our behavior so that we injure or are injured less in the future. After we have gained this knowledge, however, there is no need to maintain our injury. The second usefulness of injury is sympathy and reparations. If you can convince someone, or a judge, that their actions injured you they can be pressed for compensation. Just as above though, you can drop your injury after being compensated adequately. There are perhaps other uses for being injured but the main point remains, there is no purpose in feeling injured beyond its possible use to you.
Let us take an example of appropriate and inappropriate ways to react to the same situation then. Assume that one of your friends lied to you.
Appropriate: You use the knowledge gained from your injury and decide to not trust that friend in similar situations again. If the lie was serious enough or a repeat offense, you might break contact completely. If it was milder and they are penitent you might inform them of your expectations of them and let them know that falling short ends the friendship. In any case you decide to drop the injury when there is nothing more to be gained.
Inappropriate: You yell, scream, and drive away a friend that you enjoy over a minor lie. In this case your injury is only hurting you because you let your sense of injury grow out of proportion. Perhaps instead you reconciled with a friend but never dropped your sense of injury. Then you are still hurting yourself. If there is something worth gaining from the injury you should pursue that goal and drop the injury when you acquire it instead of letting the injury fester. If there isn't anything to gain then maintaining the injury is only driving you from your friend and negatively impacting your happiness.
To summarize, you are in control of your own temperament. Whether you forgive, forget, or bear a grudge is up to you. Any choice you make should be directed towards your own happiness though. There is no need to forgive someone who will simply hurt you again and there is no need to bear a grudge against the people you love either.