When we forgive another, we consciously choose to no longer hold their offense against them (we let go of all of the anger, and all of the other things that un-forgiveness brings about.) It sets the one who has sought our forgiveness free to move forward, and it also sets us free to move forward. That's not to say, however, that we aren't to be wise in our decisions as to how we deal with the one we've forgiven. Trust is something one earns by example.
When we forgive another (especially for grievous past offenses), what we need to do is allow them to prove to us that they can be trustworthy by their example (what they do from that time forward.) Over time, they will gain more and more of our trust (or not.) Over time, the past, with all its hurts and shortcomings, has a way of fading away.