Im not sure how much help I can be with this..
I can forgive someone.. but I do have trouble forgetting.
Example: Someone borrows money from me, intentions to pay it back may or may not have been there, but it never does get paid back. While I DO forgive them, and I don't hold that grudge against them (i.e. it no longer affects our relationship, friendship, etc.), I'm not going to loan them money again.
Another example: In a relationship, someone has lied to me.. and while I forgave them for that, they thought that meant I would never EVER look for signs of being lied to again. While I didn't go crazy and start accusing them without signs and signals, when I started to see them again, they got all up in arms about it as if it were a lack of trust.
I don't know if this is the best way to do things, it probably isn't, but it allows me to survive and maintain connections with people.