Not only might anger return after you forgive, it is likely and so please do not be thrown by this when it happens. We are imperfect and so we forgive imperfectly.
When Suzanne Freedman and I studied incest survivors who forgave their perpetrators, one important observation that we eventually made was that the courageous incest survivors, once they had forgiven, did not have high scores on the forgiveness inventory which we gave to them. Instead, they tended to get average (not outstanding) scores on forgiveness. Yet, this was sufficient for them to become emotionally healed (depression and anxiety were significantly reduced and self-esteem and hope for the future significantly rose). They still had some anger, but it was not unhealthy anger, the kind that can poison a person???s psychological insides.
Do you have to start over again if the anger returns? It depends on whether or not the anger is now the unhealthy kind (causing you to lose energy and sleep and making you irritable in your relationships) or whether it is healthy anger (motivating you to do good in the world). If it seems to be unhealthy anger, then, yes, start the process of forgiveness again with this same person and same injustice. In all likelihood, your second time forgiving may be easier and quicker than the first; the third time easier and quicker than the second.
To have to persevere in forgiving is not dishonorable nor is it a sign of failure. Instead it is a sign of courage and perseverance in the face of difficulty.