There are two basic reasons why someone would remind you of the wrongdoing: 1) The person is now interested in helping you correct a behavior still in need of correction, or 2) He or she is still quite angry, despite the proclamation of forgiveness.
With regard to point #1, I ask you to examine your behaviors to see if the ones that led to your seeking forgiveness are still present. If so, then the person’s reminders are charitable, intended to help you change.
Yet, you used the word “constantly,” which suggests to me that point #2 is actually operating here, not point #1. If this is the case, then you have to approach the person with the understanding that he or she might think forgiveness was offered to you, but it has not been offered in any complete form. Perhaps the person has taken some important steps in this direction, and I urge you to try to see this first if it is the case. Then, with this perspective (that the person might at least be trying to forgive you), I recommend the following:
1) When he or she brings up the transgression again, you should work on forgiving the person first before approaching him or her.
2) When you approach the person, you could say something like this, “I notice that you forgive me for what I did, but you keep bringing it up. This is making it hard for me to move on in a dignified way. Is it possible that you are still angry with what I did? Is it possible that you need to forgive me more deeply?”
If he or she has anger left over, please be ready for a response that could be defensive (“Oh, no, I am not angry any more”) or confrontational (“What do you mean? You are too sensitive.”) In either of these cases, you may need to practice forgiving this person for the continued anger and his or her inconsistent message of forgiveness. You may have to repeat this pattern for a little while: forgive, approach the person gently about the issue, forgive again, and approach again.