“Is it harder to forgive if a person is filled with anger compared with another person who is filled with pain and sorrow after being treated unfairly?”
It seems to me that if the anger is very intense and includes resentment or even hatred, then, yes, it is harder to forgive. Some people who are fuming with anger cannot even use the word “forgiveness” because it intensifies the anger. At the same time, if a person has deep sorrow, sometimes there is an accompanying lack of energy and the person needs some time to mourn first. At such times, the person needs to be gentle with the self as emotional healing takes place.
For additional information, see If I forgive, might I repress my angry feelings? (FAQ-5.)