Revolutionary Road

In 1955, Frank Wheeler (Leonardo DiCaprio) meets April (Kate Winslet) at a party. He is a longshoreman, hoping to be a cashier; she wants to be an actress. Frank later secures a sales position with Knox Machines, for which his father worked for twenty years, and he and April marry. The Wheelers move to 115 Revolutionary Road in suburban Connecticut when April becomes pregnant.
The couple becomes close friends with their realtor Helen Givings (Kathy Bates) and her husband Howard Givings (Richard Easton), and neighbor Milly Campbell (Kathryn Hahn) and her husband Shep (David Harbour). To their friends, the Wheelers are the perfect couple, but their relationship is troubled. April fails to make a career out of acting, while Frank hates the tedium of his work. Meanwhile, Helen asks the couple to meet her son, John (Michael Shannon), who had been declared insane, to try to help better his condition. They accept.
April wants new scenery and a chance to support the family so that Frank can find his passion, and so suggests that they move to Paris to start a new life away from the "hopeless emptiness" of their repetitive lifestyle.
The Givings (including John) talk with April and Frank. During the conversation, the Wheelers tell the Givings about their plans to live in Paris. Surprisingly, the only person that seems to comprehend their decision is John.
As the couple prepares to move, they are forced to reconsider. Frank is offered a promotion, and April becomes pregnant again. When Frank discovers she is contemplating having an abortion, he is furious and starts screaming at April, leading to a serious altercation, in which April says that they had their second child only to prove the first child was not a "mistake".
The next day, Frank takes the promotion and tries to accept his uneventful life. At the end of an evening at a jazz bar with the Campbells, Shep and April end up alone together and have sex in the car. Shep professes his long-held love for April, but she rejects his interest.
The following day, Frank confesses to having had an affair with a female assistant at his office, hoping to reconcile with April. To his surprise, April responds apathetically and tells him it does not matter as her love for him has gone, which he does not believe. The Givings come over for dinner, and Frank announces to the guests that their plans have changed, as April is pregnant. John harshly lambasts Frank for crushing April's hope as well as his acceptance of his circumstances. Angered, Frank nearly attacks John, and the Givings hurry out. Afterwards, Frank and April have a severe verbal altercation, after which April flees the house.
Frank spends the night in a drunken stupor but is shocked to find April in the kitchen the next morning, calmly making breakfast. April's mood seems to have improved, but after bidding goodbye to Frank, she breaks down and prepares to perform her own vacuum aspiration abortion, which proves fatal. Shep goes to the hospital to support Frank, who hysterically tells him, "she did it to herself." April dies in the hospital due to complications following the abortion.
Frank, feeling deep guilt and now a hollow shell of his former self, moves to the city and starts selling computers. He spends all of his extra time with his children. A new couple, the Braces, buy the house and we hear Milly telling the story of the Wheelers to them. Shep stops the story and walks out of the house, crying. When Milly asks Shep what's wrong, he asks her in return to never talk about the Wheelers ever again.
Helen talks to her husband years later about how the Braces seem to be the best-suited couple for the Wheeler's old house. When her husband mentions the Wheelers, Helen starts to talk about why she did not like them. As she continues talking about all of the things that she did not like about them, her husband turns off his hearing aid.

Question: What does Frank confess?
Answer: He has had  an affair with a female assistant at his office.