Forgiveness is a Practice
When we forgive, it is never absolute. The reason for forgiveness' transient nature lies in the fact that a misstep in the past has rippling effects in our present and possibly in our future. Every action has consequences. That sin of the past will manifest its consequences once, twice or maybe thousands of times for the rest of our lives.
When the consequence of that sin rears its ugly head, our job is to remember that we have already forgiven this sin in the past and that choose to forgive again in the present.
Does this mean we should forget about the sin? It's impossible to do so because the consequence will remind us of the sin. That sounds like a curse but it really isn't because if we forget about the sin, how can we choose differently in the present moment? Forgiveness without reform is not forgiveness; that is just mere insanity.