Day 4: Something you have to forgive someone else for

This is still a touchy subject for me. They say forgive, but don't forget, and I've been trying... I've been trying for years. I'm going to be vague to protect the innocent, but there's a chance that people who know me might still know who/what I'm talking about.
Let's just say at some point in my past, someone very dear to me had relations with a mutual friend of ours. This person withheld information about certain events, information that if it had been divulged this incident would not have happened. At least I like to think it wouldn't. I'm not sure whether I'm more upset about the actual act, or the information that was withheld, but combined it made for a less than ideal situation. OK, no sugar coating... it made for a pretty hurtful situation, one that still hurts to this day.
While I can say the past is the past and try to look past this, and can even say with almost full certainty that this type of situation would not happen again, it still leaves a pit in my stomach when I think about it. It has been several years and I wonder if this is something I'll ever truly be able to forgive...