I know you are dealing with a person who has psychological problems, so you do what you think best. Generally, however, it is best to let them know that you know about the affair. Bringing it out into the light of day takes away a lot of its magic. But don't just run off telling her you forgive her for it. Forgiveness is only worth something to her after she asks for it. Otherwise it is just more of your emotional stuff being thrown at her.
