What you are describing is a natural response to what had taken place. Initially it makes sense to feel shock, then anger then grief and acceptance. Trusting someone who had broken your trust is not easy especially when it is your husband of 36 years.
He may be sincere of his desire to remain in the marriage and you are wondering about his reason(s) for this.
You have done nothing wrong but it is what women feel when their husband had been unfaithful. They feel that they are to blame somehow and feel rejected and unattractive and undesired. The truth remains though, that he had made a decision to do something and had acted out on it.
Perhaps you could inquire if marital counseling is something he would be willing to engage in with you. An objective counselor can help you two sort out feelings, fears and plans for the future.