Describe the memory that was painful to Brian in Hatchet.

At the beginning of the novel, Brian mentions "The Secret," which continually haunts his memories and is the cause of his parents' divorce. Brian mentions that his mother was the one who initiated the divorce and says that his father did not understand why she wanted to leave. Brian then says that "The Secret" was the reason his mother wanted to end her marriage.
The painful memory that Brian recalls is explained towards the beginnings of chapters 4 and 7. Brian remembers riding his bike with his friend Terry past the Amber Mall and briefly seeing his mother in a strange station wagon. Brian then recalls seeing his mother sitting next to a short man with blond hair. In chapter 7, Brian also mentions that he remembers seeing his mother lean over the seat and kiss the blond man on the lips. Essentially, Brian witnesses his mother kissing a stranger and does not tell either of his parents that he knows the real reason behind their divorce, which is infidelity on his mother's part.