Even if you truly and sincerely forgave your spouse and rebuilt your relationship, resentment often lingers on. A blow is that is hard to forget, and many find that the memory of the affair haunts them decades after it happened.
Resentment is a normal reaction. A betrayed spouse has to deal with the unbearable memories of the pain and the lies. However, when there is no longer danger for another affair and the marriage was successfully rebuilt, this reaction might ruin the reconciliation.
We cannot actually forget what happened. But we can overcome the resentment. It fades over time as long as nothing similar happens and both spouses learn to build a physically and emotionally satisfying relationship.