Well, first of all in this particular situation we can first focus on ourselves and try to do forgiveness meditation for ourselves, rather than focusing on that person. If you were disappointed in your own conduct than at that time you didn't have the equanimity or wisdom that you would have preferred to have. So first of all deal with your own sense of ,"I didn't do so well in this" to forgive yourself. Leave the other person for now and just focus on yourself, so that you can understand, "Oh, I had a particular hindrance or reacted in a way that I wasn't happy with." At that particular time perhaps you didn't have the causes and conditions or the spiritual friend or whatever it was that may have helped you react in a skillful way. Try to see that the person you were then is not the person you are now. If you regret the action, then forgive yourself first.
Then later perhaps in the forgiveness meditation, you can focus on the other person. If that person was doing things that were unskillful, then the process is to learn first to have compassion for them, understand what hindrance and ignorance brought their actions, speech or whatever was difficult about. Try to see their ignorance, have compassion for them and forgive the ignorance of the person.
If the action was not a skillful action we do not have to forgive the action. This is where it is very difficult for many people, to bring this forgiveness, because they don't separate the action from the person. They feel that by forgiving the person they have to justify the action, that somehow it was skillful, but it wasn't. So, leave that aside. We see with wisdom that an unskillful action is unskillful and we bring our attention to the ignorance in the person. If they haven't come to regret some of their actions, that makes it more difficult to bring forgiveness. However, by seeing our own resistance to forgive others helps us to understand that this resistance brings suffering to ourselves. That understanding will help us forgive the person within ourselves. But it doesn't mean we express forgiveness verbally to this person, because that needs wisdom to caution it. If they haven't shown any regret for their action, then voicing forgiveness may only encourage that person to abuse us. So we can forgive internally, not forgiving the action, because it was an unskillful action, but forgive the person. By forgiving in this way, we can let go of Dukkha within ourselves and understand that they are making negative Kamma for themselves and they will have to receive the results for that.