Final answer:
Tim is likely experiencing repression, an ego defense mechanism where anxiety-causing memories are blocked from consciousness to protect the ego.
Step-by-step explanation:
Tim's forgetting of the girl to whom he was once engaged, who suddenly left him for another man, is an ego defense mechanism known as repression. This mechanism involves the anxiety-causing memories being blocked from consciousness as a way to protect the individual's ego. Repression is likened to turning up the radio to avoid hearing a strange noise in a car, in that the troubling memory is pushed out of conscious awareness, thus 'forgotten', at least temporarily.