Forgiving does not mean that we invite someone who hurt us once to hurt us again.
On Tuesday nights in the fall I lead a group for women who were sexually abused as children. At times I know there is no more sacred room on the planet to be in than that one. The raw emotion, honesty, pain, suffering, illumination and grace reach deeply into my soul. You can imagine forgiveness is a challenging topic in such a group.
One key point we make is that forgiveness is different than trust or reconciliation. We say, "For your own benefit it is essential that eventually you forgive your abuser. But your abuser may not be a safe person for you to be in relationship with. Forgiving does not mean you immediately trust this person will all of who you are. It does not mean you have to trust him or her with any of who you are. He may not be trustworthy. If possible we want to move towards reconciliation, but much depends on his response to hearing your story."
With infidelity recovery I often see the one who strayed demand to be trusted right away or soon after that. Often this demand includes some subtle spiritual manipulation, "You said you forgive me then you should trust if that is really true. If you really forgave me you wouldn't doubt that I'm telling the truth."
The problem is he or she just acted in a non-trustworthy manner.
So trust and forgiveness are two different animals. Forgiveness is for the one betrayed to eventually offer to the one who strayed. Trust is for the one who strayed to earn through long term consistency and consideration for the feelings and desires of the one who was betrayed.