Smart people do dumb, inconsiderate and painful things to another person and affairs have ways of luring even the smartest people into acting in ways that they never thought were possible.
When people get their normal thinking ability back, they often wonder how they could have made the choices and decisions that they did and find it hard to forgive themselves for acting so irrationally and destructively to their marriage and their spouse.
This may not be the first emotion to surface. Sometimes it is much later in the process as those who have affairs may spend a lot of energy defending their actions and blaming their spouse for the affair. Nevertheless, when they really work hard on themselves and their marriage, then reality settles in and the effect that their behavior has had on their spouse becomes more prevalent.