In my book, Forgiveness Is a Choice, I make a distinction between healthy and unhealthy anger. The healthy variety energizes a person to take action and to seek justice. Unhealthy anger is the kind that turns into resentment and abides deep within a person for long periods. Even within the category of unhealthy anger, there are resentments that are more intense than others.
As a general observation, I have seen that the deeper the anger and the more unhealthy it is, the longer forgiveness can take. This does not mean that people with profound unhealthy anger cannot be emotionally healed. On the contrary, we have worked with people who have moderate to severe depression and this has ceased at the end of treatment and has stayed away at follow-up testing. In the case of the incest survivor study the depression had stayed away at a 14-month follow-up.
So, the short answer is that forgiveness therapy can take longer when deeper anger is present and so the person needs patience and perseverance to overcome that anger. The process of forgiveness itself is not altered when there is profound anger. The time required is the key.