I feel sorry for Mellie because what happened to her was horrible, but I feel that her political career was more important to her because if she told Fitz what his father did to her, Fitz probably would have not entered the race for POTUS. Back in season 2 when Fitz spoke about how they met, he realized that it was set up by their fathers, and I believe that Mellie knew the real reason for the meeting. To her, pretending is what is real in a marriage. Also, if her marriage was so important to her, why would she bring her husband's mistress into their lives, because I believe to her being FLOTUS is all that matters to her.