As the late Lewis Smedes used to say, forgiveness is an imperfect activity for imperfect people. Even if anger surfaces occasionally, please do not grow discouraged. You can forgive again and it likely will take less time than previously and lead to better results. The idea of “permanent” forgiveness is not necessarily going to happen in all people for all circumstances. Having some anger left over happens to many people, especially when the injustice is deep. So, please be gentle with yourself and please do not expect absolute perfection as you grow in the moral virtue of forgiveness.
For additional information, see Forgiveness for Couples.