You Are Mine (1978)

Horticulturist Teresita (Nora Aunor) and Rex (Christopher De Leon), a skydiving enthusiast and passenger-jeep manufacturer, have been sweethearts for five years. At a business meeting, Rex meets vivacious but fidgety Sandra (Vilma Santos), an artist who needs tranquilizers for her nerves. She is smitten with Rex and starts going after him at his work, at home, and at his jump site until they have an affair. With the perceptive Teresita immediately sensing this and Sandra feeling guilty, Rex is unsettled by his conflicting feelings for the two.